PART III.

CHEMICAL COLOURS. 486. We give this denomination to colours which we can produce, and more or less fix, in certain bodies; which we can render more intense, which we can again take away and communicate to other bodies, and to which, therefore, we ascribe a certain permanency: duration is their prevailing characteristic. 487. In this view the chemical colours were formerly distinguished with various epithets; they were called _colores proprii, corporei, materiales, veri, permanentes, fixi_. 488. In the preceding chapter we observed how the fluctuating and transient nature of the physical colours becomes gradually fixed, thus forming the natural transition to our present subject. 489. Colour becomes fixed in bodies more or less permanently; superficially, or thoroughly. 490. All bodies are susceptible of colour; it can either be excited, rendered intense, and gradually fixed in them, or at least communicated to them. XXXIV. CHEMICAL CONTRAST. 491. In the examination of coloured appearances we had occasion everywhere to take notice of a principle of contrast: so again, in approaching the precincts of chemistry, we find a chemical contrast of a remarkable nature. We speak here, with reference to our present purpose, only of that which is comprehended under the general names of acid and alkali. 492. We characterised the chromatic contrast, in conformity with all other physical contrasts as a _more_ and _less_; ascribing the _plus_ to the yellow side, the _minus_ to the blue; and we now find that these two divisions correspond with the chemical contrasts. The yellow and yellow-red affect the acids, the blue and blue-red the alkalis; thus the phenomena of chemical colours, although still necessarily mixed up with other considerations, admit of being traced with sufficient simplicity. 493. The principal phenomena in chemical colours are produced by the oxydation of metals, and it will be seen how important this consideration is at the outset. Other facts which come into the account, and which are worthy of attention, will be examined under separate heads; in doing this we, however, expressly state that we only propose to offer some preparatory suggestions to the chemist in a very general way, without entering into the nicer chemical problems and questions, or presuming to decide on them. Our object is only to give a sketch of the mode in which, according to our conviction, the chemical theory of colours may be connected with general physics. XXXV. WHITE. 494. In treating of the dioptrical colours of the first class (155) we have already in some degree anticipated this subject. Transparent substances may be said to be in the highest class of inorganic matter. With these, colourless semi-transparence is closely connected, and white may be considered the last opaque degree of this. 495. Pure water crystallised to snow appears white, for the transparence of the separate parts makes no transparent whole. Various crystallised salts, when deprived to a certain extent of moisture, appear as a white powder. The accidentally opaque state of a pure transparent substance might be called white; thus pounded glass appears as a white powder. The cessation of a combining power, and the exhibition of the atomic quality of the substance might at the same time be taken into the account. 496. The known undecomposed earths are, in their pure state, all white. They pass to a state of transparence by natural crystallization. Silex becomes rock-crystal; argile, mica; magnesia, talc; calcareous earth and barytes appear transparent in various spars.--Note T. 497. As in the colouring of mineral bodies the metallic oxydes will often invite our attention, we observe, in conclusion, that metals, when slightly oxydated, at first appear white, as lead is converted to white lead by vegetable acid. XXXVI. BLACK. 498. Black is not exhibited in so elementary a state as white. We meet with it in the vegetable kingdom in semi-combustion; and charcoal, a substance especially worthy of attention on other accounts, exhibits a black colour. Again, if woods--for example, boards, owing to the action of light, air, and moisture, are deprived in part of their combustibility, there appears first the grey then the black colour. So again, we can convert even portions of animal substance to charcoal by semi-combustion. 499. In the same manner we often find that a sub-oxydation takes place in metals when the black colour is to be produced. Various metals, particularly iron, become black by slight oxydation, by vinegar, by mild acid fermentations; for example, a decoction of rice, &c. 500. Again, it may be inferred that a de-oxydation may produce black. This occurs in the preparation of ink, which becomes yellow by the solution of iron in strong sulphuric acid, but when partly de-oxydised by the infusion of gall-nuts, appears black. XXXVII. FIRST EXCITATION OF COLOUR. 501. In the division of physical colours, where semi-transparent mediums were considered, we saw colours antecedently to white and black. In the present case we assume a white and black already produced and fixed; and the question is, how colour can be excited in them? 502. Here, too, we can say, white that becomes darkened or dimmed inclines to yellow; black, as it becomes lighter, inclines to blue.--Note U. 503. Yellow appears on the active (plus) side, immediately in the light, the bright, the white. All white surfaces easily assume a yellow tinge; paper, linen, wool, silk, wax: transparent fluids again, which have a tendency to combustion, easily become yellow; in other words they easily pass into a very slight state of semi-transparence. 504. So again the excitement on the passive side, the tendency to obscure, dark, black, is immediately accompanied with blue, or rather with a reddish-blue. Iron dissolved in sulphuric acid, and much diluted with water, if held to the light in a glass, exhibits a beautiful violet colour as soon as a few drops only of the infusion of gall-nuts are added. This colour presents the peculiar hues of the dark topaz, the _orphninon_ of a burnt-red, as the ancients expressed it. 505. Whether any colour can be excited in the pure earths by the chemical operations of nature and art, without the admixture of metallic oxydes, is an important question, generally, indeed, answered in the negative. It is perhaps connected with the question--to what extent changes may be produced in the earths through oxydation? 506. Undoubtedly the negation of the above question is confirmed by the circumstance that wherever mineral colours are found, some trace of metal, especially of iron, shows itself; we are thus naturally led to consider how easily iron becomes oxydised, how easily the oxyde of iron assumes different colours, how infinitely divisible it is, and how quickly it communicates its colour. It were to be wished, notwithstanding, that new experiments could be made in regard to the above point, so as either to confirm or remove any doubt. 507. However this may be, the susceptibility of the earths with regard to colours already existing is very great; aluminous earth is thus particularly distinguished. 508. In proceeding to consider the metals, which in the inorganic world have the almost exclusive prerogative of appearing coloured, we find that, in their pure, independent, natural state, they are already distinguished from the pure earths by a tendency to some one colour or other. 509. While silver approximates most to pure white,--nay, really represents pure white, heightened by metallic splendour,--steel, tin, lead, and so forth, incline towards pale blue-grey; gold, on the other hand, deepens to pure yellow, copper approaches a red hue, which, under certain circumstances, increases almost to bright red, but which again returns to a yellow golden colour when combined with zinc. 510. But if metals in their pure state have so specific a determination towards this or that exhibition of colour, they are, through the effect of oxydation, in some degree reduced to a common character; for the elementary colours now come forth in their purity, and although this or that metal appears to have a particular tendency to this or that colour, we find some that can go through the whole circle of hues, others, that are capable of exhibiting more than one colour; tin, however, is distinguished by its comparative inaptitude to become coloured. We propose to give a table hereafter, showing how far the different metals can be more or less made to exhibit the different colours. 511. When the clean, smooth surface of a pure metal, on being heated, becomes overspread with a mantling colour, which passes through a series of appearances as the heat increases, this, we are persuaded, indicates the aptitude of the metal to pass through the whole range of colours. We find this phenomenon most beautifully exhibited in polished steel; but silver, copper, brass, lead, and tin, easily present similar appearances. A superficial oxydation is probably here taking place, as may be inferred from the effects of the operation when continued, especially in the more easily oxydizable metals. 512. The same conclusion may be drawn from the fact that iron is more easily oxydizable by acid liquids when it is red hot, for in this case the two effects concur with each other. We observe, again, that steel, accordingly as it is hardened in different stages of its colorification, may exhibit a difference of elasticity: this is quite natural, for the various appearances of colour indicate various degrees of heat.[1] 513. If we look beyond this superficial mantling, this pellicle of colour, we observe that as metals are oxydized throughout their masses, white or black appears with the first degree of heat, as may be seen in white lead, iron, and quicksilver. 514. If we examine further, and look for the actual exhibition of colour, we find it most frequently on the _plus_ side. The mantling, so often mentioned, of smooth metallic surfaces begins with yellow. Iron passes presently into yellow ochre, lead from white lead to massicot, quicksilver from æthiops to yellow turbith. The solutions of gold and platinum in acids are yellow. 515. The exhibitions on the _minus_ side are less frequent. Copper slightly oxydized appears blue. In the preparation of Prussian-blue, alkalis are employed. 516. Generally, however, these appearances of colour are of so mutable a nature that chemists look upon them as deceptive tests, at least in the nicer gradations. For ourselves, as we can only treat of these matters in a general way, we merely observe that the appearances of colour in metals may be classed according to their origin, manifold appearance, and cessation, as various results of oxydation, hyper-oxydation, ab-oxydation, and de-oxydation.[2] [1] See par. 478. [2] As these terms are afterwards referred to (par. 525), it was necessary to preserve them. XXXVIII. AUGMENTATION OF COLOUR.[1] 517. The augmentation of colour exhibits itself as a condensation, a fulness, a darkening of the hue. We have before seen, in treating of colourless mediums, that by increasing the degree of opacity in the medium, we can deepen a bright object from the lightest yellow to the intensest ruby-red. Blue, on the other hand, increases to the most beautiful violet, if we rarefy and diminish a semi-opaque medium, itself lighted, but through which we see darkness (150, 151). 518. If the colour is positive, a similar colour appears in the intenser state. Thus if we fill a white porcelain cup with a pure yellow liquor, the fluid will appear to become gradually redder towards the bottom, and at last appears orange. If we pour a pure blue solution into another cup, the upper portion will exhibit a sky-blue, that towards the bottom, a beautiful violet. If the cup is placed in the sun, the shadowed side, even of the upper portion, is already violet. If we throw a shadow with the hand, or any other substance, over the illumined portion, the shadow in like manner appears reddish. 519. This is one of the most important appearances connected with the doctrine of colours, for we here manifestly find that a difference of quantity produces a corresponding qualified impression on our senses. In speaking of the last class of epoptical colours (452, 485), we stated our conjecture that the colouring of steel might perhaps be traced to the doctrine of the semi-transparent mediums, and we would here again recall this to the reader's recollection. 520. All chemical augmentation of colour, again, is the immediate consequence of continued excitation. The augmentation advances constantly and unremittingly, and it is to be observed that the increase of intenseness is most common on the _plus_ side. Yellow iron ochre increases, as well by fire as by other operations, to a very strong red: massicot is increased to red lead, turbith to vermilion, which last attains a very high degree of the yellow-red. An intimate saturation of the metal by the acid, and its separation to infinity, take place together with the above effects. 521. The augmentation on the _minus_ side is less frequent; but we observe that the more pure and condensed the Prussian-blue or cobalt glass is prepared, the more readily it assumes a reddish hue and inclines to the violet. 522. The French have a happy expression for the less perceptible tendency of yellow and blue towards red: they say the colour has "un œil de rouge," which we might perhaps express by a reddish glance (einen röthlichen blick). [1] Steigerung, literally _gradual ascent_. See the note to par. 523. XXXIX. CULMINATION[1] 523. This is the consequence of still progressing augmentation. Red, in which neither yellow nor blue is to be detected, here constitutes the acme. 524. If we wish to select a striking example of a culmination on the _plus_ side, we again find it in the coloured steel, which attains the bright red acme, and can be arrested at this point. 525. Were we here to employ the terminology before proposed, we should say that the first oxydation produces yellow, the hyper-oxydation yellow-red; that here a kind of maximum exists, and that then an ab-oxydation, and lastly a de-oxydation takes place. 526. High degrees of oxydation produce a bright red. Gold in solution, precipitated by a solution of tin, appears bright red: oxyde of arsenic, in combination, with sulphur, produces a ruby colour. 527. How far, however, a kind of sub-oxydation may co-operate in some culminations, is matter for inquiry; for an influence of alkalis on yellow-red also appears to produce the culmination; the colour reaching the acme by being forced towards the _minus_ side. 528. The Dutch prepare a colour known by the name of vermilion, from the best Hungarian cinnabar, which exhibits the brightest yellow-red. This vermilion is still only a cinnabar, which, however, approximates the pure red, and it may be conjectured that alkalis are used to bring it nearer to the culminating point. 529. Vegetable juices, treated in this way, offer very striking examples of the above effects. The colouring-matter of turmeric, annotto, dyer's saffron,[2] and other vegetables, being extracted with spirits of wine, exhibits tints of yellow, yellow-red, and hyacinth-red; these, by the admixture of alkalis, pass to the culminating point, and even beyond it to blue-red. 530. No instance of a culmination on the _minus_ side has come to my knowledge in the mineral and vegetable kingdoms. In the animal kingdom the juice of the murex is remarkable; of its augmentation and culmination on the _minus_ side, we shall hereafter have occasion to speak. [1] _Culmination_, the original word. It might have been rendered _maximum of colour_, but as the author supposes an _ascent_ through yellow and blue to red, his meaning is better expressed by his own term. [2] Curcuma, Bixa Orellana, Carthamus Tinctorius. XL. FLUCTUATION. 531. The mutability of colour is so great, that even those pigments, which may have been considered to be defined and arrested, still admit of slight variations on one side or the other. This mutability is most remarkable near the culminating point, and is effected in a very striking manner by the alternate employment of acids and alkalis. 532. To express this appearance in dyeing, the French make use of the word "virer," to turn from one side to the other; they thus very adroitly convey an idea which others attempt to express by terms indicating the component hues. 533. The effect produced with litmus is one of the most known and striking of this kind. This colouring substance is tendered red-blue by means of alkalis. The red-blue is very readily changed to red-yellow by means of acids, and again returns to its first state by again employing alkalis. The question whether a culminating point is to be discovered and arrested by nice experiments, is left to those who are practised in these operations. Dyeing, especially scarlet-dyeing, might afford a variety of examples of this fluctuation. XLI. PASSAGE THROUGH THE WHOLE SCALE. 534. The first excitation and gradual increase of colour take place more on the _plus_ than on the _minus_ side. So, also, in passing through the whole scale, colour exhibits itself most on the _plus_ side. 535. A passage of this kind, regular and evident to the senses, from yellow through red to blue, is apparent in the colouring of steel. 536. The metals may be arrested at various points of the colorific circle by various degrees and kinds of oxydation. 537. As they also appear green, a question arises whether chemists know any instance in the mineral kingdom of a constant transition from yellow, through green, to blue, and _vice versâ_. Oxyde of iron, melted with glass, produces first a green, and with a more powerful heat, a blue colour. 538. We may here observe of green generally, that it appears, especially in an atomic sense, and certainly in a pure state, when we mix blue and yellow: but, again, an impure and dirty yellow soon gives us the impression of green; yellow and black already produce green; this, however, is owing to the affinity between black and blue. An imperfect yellow, such as that of sulphur, gives us the impression of a greenish hue: thus, again, an imperfect blue appears green. The green of wine bottles arises, it appears, from an imperfect union of the oxyde of iron with the glass. If we produce a more complete union by greater heat, a beautiful blue-glass is the result. 539. From all this it appears that a certain chasm exists in nature between yellow and blue, the opposite characters of which, it is true, may be done away atomically by due immixture, and, thus combined, to green; but the true reconciliation between yellow and blue, it seems, only takes place by means of red. 540. The process, however, which appears unattainable in inorganic substances, we shall find to be possible when we turn our attention to organic productions; for in these, the passage through the whole circle from yellow, through green and blue, to red, really takes place. XLII. INVERSION. 541. Again, an immediate inversion or change to the totally opposite hue, is a very remarkable appearance which sometimes occurs; at present, we are merely enabled to adduce what follows. 542. The mineral chameleon, a name which has been given to an oxyde of manganese, may be considered, in its perfectly dry state, as a green powder. If we strew it in water, the green colour displays itself very beautifully in the first moment of solution, but it changes presently to the bright red opposite to green, without any apparent intermediate state. 543. The same occurs with the sympathetic ink, which may be considered a reddish liquid, but which, when dried by warmth, appears as a green colour on paper. 544. In fact, this phenomenon appears to be owing to the conflict between a dry and moist state, as has been already observed, if we are not mistaken, by the chemists. We may look to the improvements of time to point out what may further be deduced from these phenomena, and to show what other facts they may be connected with. XLIII. FIXATION. 545. Mutable as we have hitherto found colour to be, even as a substance, yet under certain circumstances it may at last be fixed. 546. There are bodies capable of being entirely converted into colouring matter: here it may be said that the colour fixes itself in its own substance, stops at a certain point, and is there defined. Such colouring substances are found throughout nature; the vegetable world affords a great quantity of examples, among which some are particularly distinguished, and may be considered as the representatives of the rest; such as, on the active side, madder, on the passive side, indigo. 547. In order to make these materials available in use, it is necessary that the colouring quality in them should be intimately condensed, and the tinging substance refined, practically speaking, to an infinite divisibility. This is accomplished in various ways, and particularly by the well-known means of fermentation and decomposition. 548. These colouring substances now attach themselves again to other bodies. Thus, in the mineral kingdom they adhere to earths and metallic oxydes; they unite in melting with glasses; and in this case, as the light is transmitted through them, they appear in the greatest beauty, while an eternal duration may be ascribed to them. 549. They fasten on vegetable and animal bodies with more or less power, and remain more or less permanently; partly owing to their nature,--as yellow, for instance, is more evanescent than blue,--or owing to the nature of the substance on which they appear. They last less in vegetable than in animal substances, and even within this latter kingdom there are again varieties. Hemp or cotton threads, silk or wool, exhibit very different relations to colouring substances. 550. Here comes into the account the important operation of employing mordants, which may be considered as the intermediate agents between the colour and the recipient substance; various works on dyeing speak of this circumstantially. Suffice it to have alluded to processes by means of which the colour retains a permanency only to be destroyed with the substance, and which may even increase in brightness and beauty by use. XLIV. INTERMIXTURE, REAL. 551. Every intermixture pre-supposes a specific state of colour; and thus when we speak of intermixture, we here understand it in an atomic sense. We must first have before us certain bodies arrested at any given point of the colorific circle, before we can produce gradations by their union. 552. Yellow, blue, and red, may be assumed as pure elementary colours, already existing; from these, violet, orange, and green, are the simplest combined results. 553. Some persons have taken much pains to define these intermixtures more accurately, by relations of number, measure, and weight, but nothing very profitable has been thus accomplished. 554. Painting consists, strictly speaking, in the intermixture of such specific colouring bodies and their infinite possible combinations--combinations which can only be appreciated by the nicest, most practised eye, and only accomplished under its influence. 555. The intimate combination of these ingredients is effected, in the first instance, through the most perfect comminution of the material by means of grinding, washing, &c., as well as by vehicles or liquid mediums which hold together the pulverized substance, and combine organically, as it were, the unorganic; such are the oils, resins, &c.--Note V. 556. If all the colours are mixed together they retain their general character as σκιερόν, and as they are no longer seen next each other, no completeness, no harmony, is experienced; the result is grey, which, like apparent colour, always appears somewhat darker than white, and somewhat lighter than black. 557. This grey may be produced in various ways. By mixing yellow and blue to an emerald green, and then adding pure red, till all three neutralize each other; or, by placing the primitive and intermediate colours next each other in a certain proportion, and afterwards mixing them. 558. That all the colours mixed together produce white, is an absurdity which people have credulously been accustomed to repeat for a century, in opposition to the evidence of their senses. 559. Colours when mixed together retain their original darkness. The darker the colours, the darker will be the grey resulting from their union, till at last this grey approaches black. The lighter the colours the lighter will be the grey, which at last approaches white. XLV. INTERMIXTURE, APPARENT. 560. The intermixture, which is only apparent, naturally invites our attention in connexion with the foregoing; it is in many respects important, and, indeed, the intermixture which we have distinguished as real, might be considered as merely apparent. For the elements of which the combined colour consists are only too small to be considered as distinct parts. Yellow and blue powders mingled together appear green to the naked eye, but through a magnifying glass we can still perceive yellow and blue distinct from each other. Thus yellow and blue stripes seen at a distance, present a green mass; the same observation is applicable with regard to the intermixture of other specific colours. 561. In the description of our apparatus we shall have occasion to mention the wheel by means of which the apparent intermixture is produced by rapid movement. Various colours are arranged near each other round the edge of a disk, which is made to revolve with velocity, and thus by having several such disks ready, every possible intermixture can be presented to the eye, as well as the mixture of all colours to grey, darker or lighter, according to the depth of the tints as above explained. 562. Physiological colours admit, in like manner, of being mixed with others. If, for example, we produce the blue shadow (65) on a light yellow paper, the surface will appear green. The same happens with regard to the other colours if the necessary preparations are attended to. 563. If, when the eye is impressed with visionary images that last for a while, we look on coloured surfaces, an intermixture also takes place; the spectrum is determined to a new colour which is composed of the two. 564. Physical colours also admit of combination. Here might be adduced the experiments in which many-coloured images are seen through the prism, as we have before shown in detail (258, 284). 565. Those who have prosecuted these inquiries have, however, paid most attention to the appearances which take place when the prismatic colours are thrown on coloured surfaces. 566. What is seen under these circumstances is quite simple. In the first place it must be remembered that the prismatic colours are much more vivid than the colours of the surface on which they are thrown. Secondly, we have to consider that the prismatic colours may be either homogeneous or heterogeneous, with the recipient surface. In the former case the surface deepens and enhances them, and is itself enhanced in return, as a coloured stone is displayed by a similarly coloured foil. In the opposite case each vitiates, disturbs, and destroys the other. 567. These experiments may be repeated with coloured glasses, by causing the sun-light to shine through them on coloured surfaces. In every instance similar results will appear. 568. The same effect takes place when we look on coloured objects through coloured glasses; the colours being thus according to the same conditions enhanced, subdued, or neutralized. 569. If the prismatic colours are suffered to pass through coloured glasses, the appearances that take place are perfectly analogous; in these cases more or less force, more or less light and dark, the clearness and cleanness of the glass are all to be allowed for, as they produce many delicate varieties of effect: these will not escape the notice of every accurate observer who takes sufficient interest in the inquiry to go through the experiments. 570. It is scarcely necessary to mention that several coloured glasses, as well as oiled or transparent papers, placed over each other, may be made to produce and exhibit every kind of intermixture at pleasure. 571. Lastly, the operation of glazing in painting belongs to this kind of intermixture; by this means a much more refined union may be produced than that arising from the mechanical, atomic mixture which is commonly employed. XLVI. COMMUNICATION, ACTUAL. 572. Having now provided the colouring materials, as before shown, a further question arises how to communicate these to colourless substances: the answer is of the greatest importance from the connexion of the object with the ordinary wants of men, with useful purposes, and with commercial and technical interests. 573. Here, again, the dark quality of every colour again comes into the account. From a yellow, that is very near to white, through orange, and the hue of minium to pure red and carmine, through all gradations of violet to the deepest blue which is almost identified with black, colour still increases in darkness. Blue once defined, admits of being diluted, made light, united with yellow, and then, as green, it approaches the light side of the scale: but this is by no means according to its own nature. 574. In the physiological colours we have already seen that they are less than the light, inasmuch as they are a repetition of an impression of light, nay, at last they leave this impression quite as a dark. In physical experiments the employment of semi-transparent mediums, the effect of semi-transparent accessory images, taught us that in such cases we have to do with a subdued light, with a transition to darkness. 575. In treating of the chemical origin of pigments we found that the same effect was produced on the very first excitement. The yellow tinge which mantles over the steel, already darkens the shining surface. In changing white lead to massicot it is evident that the yellow is darker than white. 576. This process is in the highest degree delicate; the growing intenseness, as it still increases, tinges the substance more and more intimately and powerfully, and thus indicates the extreme fineness, and the infinite divisibility of the coloured atoms. 577. The colours which approach the dark side, and consequently, blue in particular, can be made to approximate to black; in fact, a very perfect Prussian blue, or an indigo acted on by vitriolic acid appears almost as a black. 578. A remarkable appearance may be here adverted to; pigments, in their deepest and most condensed state, especially those produced from the vegetable kingdom, such as the indigo just mentioned, or madder carried to its intensest hue, no longer show their own colour; on the contrary, a decided metallic shine is seen on their surface, in which the physiological compensatory colour appears. 579. All good indigo exhibits a copper-colour in its fracture, a circumstance attended to, as a known characteristic, in trade. Again, the indigo which has been acted on by sulphuric acid, if thickly laid on, or suffered to dry so that neither white paper nor the porcelain can appear through, exhibits a colour approaching to orange. 580. The bright red Spanish rouge, probably prepared from madder, exhibits on its surface a perfectly green, metallic shine. If this colour, or the blue before mentioned, is washed with a pencil on porcelain or paper, it is seen in its real state owing to the bright ground shining through. 581. Coloured liquids appear black when no light is transmitted through them, as we may easily see in cubic tin vessels with glass bottoms. In these every transparent-coloured infusion will appear black and colourless if we place a black surface under them. 582. If we contrive that the image of a flame be reflected from the bottom, the image will appear coloured. If we lift up the vessel and suffer the transmitted light to fall on white paper under it, the colour of the liquid appears on the paper. Every light ground seen through such a coloured medium exhibits the colour of the medium. 583. Thus every colour, in order to be seen, must have a light within or behind it. Hence the lighter and brighter the grounds are, the more brilliant the colours appear. If we pass lac-varnish over a shining white metal surface, as the so-called foils are prepared, the splendour of the colour is displayed by this internally reflected light as powerfully as in any prismatic experiment; nay, the force of the physical colours is owing principally to the circumstance that light is always acting with and behind them. 584. Lichtenberg, who of necessity followed the received theory, owing to the time and circumstances in which he lived, was yet too good an observer, and too acute not to explain and classify, after his fashion, what was evident to his senses. He says, in the preface to Delaval, "It appears to me also, on other grounds, probable, that our organ, in order to be impressed by a colour, must at the same time be impressed by all light (white)." 585. To procure white as a ground is the chief business of the dyer. Every colour may be easily communicated to colourless earths, especially to alum: but the dyer has especially to do with animal and vegetable products as the ground of his operations. 586. Everything living tends to colour--to local, specific colour, to effect, to opacity--pervading the minutest atoms. Everything in which life is extinct approximates to white (494), to the abstract, the general state, to clearness[1], to transparence. 587. How this is put in practice in technical operations remains to be adverted to in the chapter on the privation of colour. With regard to the communication of colour, we have especially to bear in mind that animals and vegetables, in a living state, produce colours, and hence their substances, if deprived of colours, can the more readily re-assume them. [1] Verklärung, literally _clarification_. XLVII. COMMUNICATION, APPARENT. 588. The communication of colours, real as well as apparent, corresponds, as may easily be seen, with their intermixture: we need not, therefore, repeat what has been already sufficiently entered into. 589. Yet we may here point out more circumstantially the importance of an apparent communication which takes place by means of reflection. This phenomenon is well known, but still it is pregnant with inferences, and is of the greatest importance both to the investigator of nature and to the painter. 590. Let a surface coloured with any one of the positive colours be placed in the sun, and let its reflection be thrown on other colourless objects. This reflection is a kind of subdued light, a half-light, a half-shadow, which, in a subdued state, reflects the colours in question. 591. If this reflection acts on light surfaces, it is so far overpowered that we can scarcely perceive the colour which accompanies it; but if it acts on shadowed portions, a sort of magical union takes place with the σκιερῷ. Shadow is the proper element of colour, and in this case a subdued colour approaches it, lighting up, tinging, and enlivening it. And thus arises an appearance, as powerful as agreeable, which may render the most pleasing service to the painter who knows how to make use of it. These are the types of the so-called reflexes, which were only noticed late in the history of art, and which have been too seldom employed in their full variety. 592. The schoolmen called these colours _colores notionales_ and _intentionales_, and the history of the doctrine of colours will generally show that the old inquirers already observed the phenomena well enough, and knew how to distinguish them properly, although the whole method of treating such subjects is very different from ours. XLVIII. EXTRACTION. 593. Colour may be extracted from substances, whether they possess it naturally or by communication, in various ways. We have thus the power to remove it intentionally for a useful purpose, but, on the other hand, it often flies contrary to our wish. 594. Not only are the elementary earths in their natural state white, but vegetable and animal substances can be reduced to a white state without disturbing their texture. A pure white is very desirable for various uses, as in the instance of our preferring to use linen and cotton stuffs uncoloured. In like manner some silk stuffs, paper, and other substances, are the more agreeable the whiter they can be. Again, the chief basis of all dyeing consists in white grounds. For these reasons manufacturers, aided by accident and contrivance, have devoted themselves assiduously to discover means of extracting colour: infinite experiments have been made in connexion with this object, and many important facts have been arrived at. 595. It is in accomplishing this entire extraction of colour that the operation of bleaching consists, which is very generally practised empirically or methodically. We will here shortly state the leading principles. 596. Light is considered as one of the first means of extracting colour from substances, and not only the sun-light, but the mere powerless day-light: for as both lights--the direct light of the sun, as well as the derived light of the sky--kindle Bologna phosphorus, so both act on coloured surfaces. Whether the light attacks the colour allied to it, and, as it were, kindles and consumes it, thus reducing the definite quality to a general state, or whether some other operation, unknown to us, takes place, it is clear that light exercises a great power on coloured surfaces, and bleaches them more or less. Here, however, the different colours exhibit a different degree of durability; yellow, especially if prepared from certain materials, is, in this case, the first to fly. 597. Not only light, but air, and especially water, act strongly in destroying colour. It has been even asserted that thread, well soaked and spread on the grass at night, bleaches better than that which is exposed, after soaking, to the sun-light. Thus, in this case, water proves to be a solving and conducting agent, removing the accidental quality, and restoring the substance to a general or colourless state. 598. The extraction of colour is also effected by re-agents. Spirits of wine has a peculiar tendency to attract the juice which tinges plants, and becomes coloured with it often in a very permanent manner. Sulphuric acid is very efficient in removing colour, especially from wool and silk, and every one is acquainted with the use of sulphur vapours in bleaching. 599. The strongest acids have been recommended more recently as more expeditious agents in bleaching. 600. The alkaline re-agents produce the same effects by contrary means--lixiviums alone, oils and fat combined with lixiviums to soap, and so forth. 601. Before we dismiss this subject, we observe [Pg 240] that it may be well worth while to make certain delicate experiments as to how far light and air exhibit their action in the removal of colour. It might be possible to expose coloured substances to the light under glass bells, without air, or filled with common or particular kinds of air. The colours might be those of known fugacity, and it might be observed whether any of the volatilized colour attached itself to the glass or was otherwise perceptible as a deposit or precipitate; whether, again, in such a case, this appearance would be perfectly like that which had gradually ceased to be visible, or whether it had suffered any change. Skilful experimentalists might devise various contrivances with a view to such researches. 602. Having thus first considered the operations of nature as subservient to our proposes, we add a few observations on the modes in which they act against us. 603. The art of painting is so circumstanced that the most beautiful results of mind and labour are altered and destroyed in various ways by time. Hence great pains have been always taken to find durable pigments, and so to unite them with each other and with their ground, that their permanency might be further insured. The technical history of the schools of painting affords sufficient information on this point. 604. We may here, too, mention a minor art, to which, in relation to dyeing, we are much indebted, namely, the weaving of tapestry. As the manufacturers were enabled to imitate the most delicate shades of pictures, and hence often brought the most variously coloured materials together, it was soon observed that the colours were not all equally durable, but that some faded from the tapestry more quickly than others. Hence the most diligent efforts were made to ensure an equal permanency to all the colours and their gradations. This object was especially promoted in France, under Colbert, whose regulations to this effect constitute an epoch in the history of dyeing. The gay dye which only aimed at a transient beauty, was practised by a particular guild. On the other hand, great pains were taken to define the technical processes which promised durability. And thus, after considering the artificial extraction, the evanescence, and the perishable nature of brilliant appearances of colour, we are again returned to the desideratum of permanency. XLIX. NOMENCLATURE. 605. After what has been adduced respecting the origin, the increase, and the affinity of colours, we may be better enabled to judge what nomenclature would be desirable in future, and what might be retained of that hitherto in use. 606. The nomenclature of colours, like all other modes of designation, but especially those employed to distinguish the objects of sense, proceeded in the first instance from particular to general, and from general back again to particular terms. The name of the species became a generic name to which the individual was again referred. 607. This method might have been followed in consequence of the mutability and uncertainty of ancient modes of expression, especially since, in the early ages, more reliance may be supposed to have been placed on the vivid impressions of sense. The qualities of objects were described indistinctly, because they were impressed clearly on every imagination. 608. The pure chromatic circle was limited, it is true; but, specific as it was, it appears to have been applied to innumerable objects, while it was circumscribed by qualifying characteristics. If we take a glance at the copiousness of the Greek and Roman terms, we shall perceive how mutable the words were, and how easily each was adapted to almost every point in the colorific circle.--Note W. 609. In modern ages terms for many new gradations were introduced in consequence of the various operations of dyeing. Even the colours of fashion and their designations, represented an endless series of specific hues. We shall, on occasion, employ the chromatic terminology of modern languages, whence it will appear that the aim has gradually been to introduce more exact definitions, and to individualise and arrest a fixed and specific state by language equally distinct. 610. With regard to the German terminology, it has the advantage of possessing four monosyllabic names no longer to be traced to their origin, viz., yellow (Gelb), blue, red, green. They represent the most general idea of colour to the imagination, without reference to any very specific modification. 611. If we were to add two other qualifying terms to each of these four, as thus--red-yellow, and yellow-red, red-blue and blue-red, yellow-green and green-yellow, blue-green and green-blue,[1] we should express the gradations of the chromatic circle with sufficient distinctness; and if we were to add the designations of light and dark, and again define, in some measure, the degree of purity or its opposite by the monosyllables black, white, grey, brown, we should have a tolerably sufficient range of expressions to describe the ordinary appearances presented to us, without troubling ourselves whether they were produced dynamically or atomically. 612. The specific and proper terms in use might, however, still be conveniently employed, and we have thus made use of the words orange and violet. We have in like manner employed the word "_purpur_" to designate a pure central red, because the secretion of the murex or "_purpura_" is to be carried to the highest point of culmination by the action of the sun-light on fine linen saturated with the juice. [1] This description is suffered to remain because it accounts for the terminology employed throughout.--T. L. MINERALS. 613. The colours of minerals are all of a chemical nature, and thus the modes in which they are produced may be explained in a general way by what has been said on the subject of chemical colours. 614. Among the external characteristics of minerals, the description of their colours occupies the first place; and great pains have been taken, in the spirit of modern times, to define and arrest every such appearance exactly: by this means, however, new difficulties, it appears to us, have been created, which occasion no little inconvenience in practice. 615. It is true, this precision, when we reflect how it arose, carries with it its own excuse. The painter has at all times been privileged in the use of colours. The few specific hues, in themselves, admitted of no change; but from these, innumerable gradations were artificially produced which imitated the surface of natural objects. It was, therefore, not to be wondered at that these gradations should also be adopted as criterions, and that the artist should be invited to produce tinted patterns with which the objects of nature might be compared, and according to which they were to receive their designations. 616. But, after all, the terminology of colours which has been introduced in mineralogy, is open to many objections. The terms, for instance, have not been borrowed from the mineral kingdom, as was possible enough in most cases, but from all kinds of visible objects. Too many specific terms have been adopted; and in seeking to establish new definitions by combining these, the nomenclators have not reflected that they thus altogether efface the image from the imagination, and the idea from the understanding. Lastly, these individual designations of colours, employed to a certain extent as elementary definitions, are not arranged in the best manner as regards their respective derivation from each other: hence, the scholar must learn every single designation, and impress an almost lifeless but positive language on his memory. The further consideration of this would be too foreign to our present subject.[1] [1] These remarks have reference to the German mineralogical terminology.--T. LI. PLANTS. 617. The colours of organic bodies in general may be considered as a higher kind of chemical operation, for which reason the ancients employed the word concoction, πέψις, to designate the process. All the elementary colours, as well as the combined and secondary hues, appear on the surface of organic productions, while on the other hand, the interior, if not colourless, appears, strictly speaking, negative when brought to the light. As we propose to communicate our views respecting organic nature, to a certain extent, in another place, we only insert here what has been before connected with the doctrine of colours, while it may serve as an introduction to the further consideration of the views alluded to: and first, of plants. 618. Seeds, bulbs, roots, and what is generally shut out from the light, or immediately surrounded by the earth, appear, for the most part, white. 619. Plants reared from seed, in darkness, are white, or approaching to yellow. Light, on the other hand, in acting on their colours, acts at the same time on their form. 620. Plants which grow in darkness make, it is true, long shoots from joint to joint: but the stems between two joints are thus longer than they should be; no side stems are produced, and the metamorphosis of the plant does not take place. 621. Light, on the other hand, places it at once in an active state; the plant appears green, and the course of the metamorphosis proceeds uninterruptedly to the period of reproduction. 622. We know that the leaves of the stem are only preparations and pre-significations of the instruments of florification and fructification, and accordingly we can already see colours in the leaves of the stem which, as it were, announce the flower from afar, as is the case in the amaranthus. 623. There are white flowers whose petals have wrought or refined themselves to the greatest purity; there are coloured ones, in which the elementary hues may be said to fluctuate to and fro. There are some which, in tending to the higher state, have only partially emancipated themselves from the green of the plant. 624. Flowers of the same genus, and even of the same kind, are found of all colours. Roses, and particularly mallows, for example, vary through a great portion of the colorific circle from white to yellow, then through red-yellow to bright red, and from thence to the darkest hue it can exhibit as it approaches blue. 625. Others already begin from a higher degree in the scale, as, for example, the poppy, which is yellow-red in the first instance, and which afterwards approaches a violet hue. 626. Yet the same colours in species, varieties, and even in families and classes, if not constant, are still predominant, especially the yellow colour: blue is throughout rarer. 627. A process somewhat similar takes place in the juicy capsule of the fruit, for it increases in colour from the green, through the yellowish and yellow, up to the highest red, the colour of the rind thus indicating the degree of ripeness. Some are coloured all round, some only on the sunny side, in which last case the augmentation of the yellow into red,--the gradations crowding in and upon each other,--may be very well observed. 628. Many fruits, too, are coloured internally; pure red juices, especially, are common. 629. The colour which is found superficially in the flower and penetratingly in the fruit, spreads itself through all the remaining parts, colouring the roots and the juices of the stem, and this with a very rich and powerful hue. 630. So, again, the colour of the wood passes from yellow through the different degrees of red up to pure red and on to brown. Blue woods are unknown to me; and thus in this degree of organisation the active side exhibits itself powerfully, although both principles appear balanced in the general green of the plant. 631. We have seen above that the germ pushing from the earth is generally white and yellowish, but that by means of the action of light and air it acquires a green colour. The same happens with young leaves of trees, as may be seen, for example, in the birch, the young leaves of which are yellowish, and if boiled, yield a beautiful yellow juice: afterwards they become greener, while the leaves of other trees become gradually blue-green. 632. Thus a yellow ingredient appears to belong more essentially to leaves than a blue one; for this last vanishes in the autumn, and the yellow of the leaf appears changed to a brown colour. Still more remarkable, however, are the particular cases where leaves in autumn again become pure yellow, and others increase to the brightest red. 633. Other plants, again, may, by artificial treatment be entirely converted to a colouring matter, which is as fine, active, and infinitely divisible as any other. Indigo and madder, with which so much is effected, are examples: lichens are also used for dyes. 634. To this fact another stands immediately opposed; we can, namely, extract the colouring part of plants, and, as it were, exhibit it apart, while the organisation does not on this account appear to suffer at all. The colours of flowers may be extracted by spirits of wine, and tinge it; the petals meanwhile becoming white. 635. There are various modes of acting on flowers and their juices by re-agents. This has been done by Boyle in many experiments. Roses are bleached by sulphur, and may be restored to their first state by other acids; roses are turned green by the smoke of tobacco. LII. WORMS, INSECTS, FISHES. 636. With regard to creatures belonging to the lower degrees of organisation, we may first observe that worms, which live in the earth and remain in darkness and cold moisture, are imperfectly negatively coloured; worms bred in warm moisture and darkness are colourless; light seems expressly necessary to the definite exhibition of colour. 637. Creatures which live in water, which, although a very dense medium, suffers sufficient light to pass through it, appear more or less coloured. Zoophytes, which appear to animate the purest calcareous earth, are mostly white; yet we find corals deepened into the most beautiful yellow-red: in other cells of worms this colour increases nearly to bright red. 638. The shells of the crustaceous tribe are beautifully designed and coloured, yet it is to be remarked that neither land-snails nor the shells of crustacea of fresh water, are adorned with such bright colours as those of the sea. 639. In examining shells, particularly such as are spiral, we find that a series of animal organs, similar to each other, must have moved increasingly forward, and in turning on an axis produced the shell in a series of chambers, divisions, tubes, and prominences, according to a plan for ever growing larger. We remark, however, that a tinging juice must have accompanied the development of these organs, a juice which marked the surface of the shell, probably through the immediate co-operation of the sea-water, with coloured lines, points, spots, and shadings: this must have taken place at regular intervals, and thus left the indications of increasing growth lastingly on the exterior; meanwhile the interior is generally found white or only faintly coloured. 640. That such a juice is to be found in shell-fish is, besides, sufficiently proved by experience; for the creatures furnish it in its liquid and colouring state: the juice of the ink-fish is an example. But a much stronger is exhibited in the red juice found in many shell-fish, which was so famous in ancient times, and has been employed with advantage by the moderns. There is, it appears, in the entrails of many of the crustaceous tribe a certain vessel which is filled with a red juice; this contains a very strong and durable colouring substance, so much so that the entire creature may be crushed and boiled, and yet out of this broth a sufficiently strong tinging liquid may be extracted. But the little vessel filled with colour may be separated from the animal, by which means of course a concentrated juice is gained. 641. This juice has the property that when exposed to light and air it appears first yellowish, then greenish; it then passes to blue, then to a violet, gradually growing redder; and lastly, by the action of the sun, and especially if transferred to cambric, it assumes a pure bright red colour. 642. Thus we should here have an augmentation, even to culmination, on the _minus_ side, which we cannot easily meet with in inorganic cases; indeed, we might almost call this example a passage through the whole scale, and we are persuaded that by due experiments the entire revolution of the circle might really be effected, for there is no doubt that by acids duly employed, the pure red may be pushed beyond the culminating point towards scarlet. 643. This juice appears on the one hand to be connected with the phenomena of reproduction, eggs being found, the embryos of future shell-fish, which contain a similar colouring principle. On the other hand, in animals ranking higher in the scale of being, the secretion appears to bear some relation to the development of the blood. The blood exhibits similar properties in regard to colour; in its thinnest state it appears yellow; thickened, as it is found in the veins, it appears red; while the arterial blood exhibits a brighter red, probably owing to the oxydation which takes place by means of breathing. The venous blood approaches more to violet, and by this mutability denotes the tendency to that augmentation and progression which are now familiar to us. 644. Before we quit the element whence we derived the foregoing examples, we may add a few observations on fishes, whose scaly surface is coloured either altogether in stripes, or in spots, and still oftener exhibits a certain iridescent appearance, indicating the affinity of the scales with the coats of shell-fish, mother-of-pearl, and even the pearl itself. At the same time it should not be forgotten that warmer climates, the influence of which extends to the watery regions, produce, embellish, and enhance these colours in fishes in a still greater degree. 645. In Otaheite, Forster observed fishes with beautifully iridescent surfaces, and this effect was especially apparent at the moment when the fish died. We may here call to mind the hues of the chameleon, and other similar appearances; for when similar facts are presented together, we are better enabled to trace them. 646. Lastly, although not strictly in the same class, the iridescent appearance of certain molluscæ may be mentioned, as well as the phosphorescence which, in some marine creatures, it is said becomes iridescent just before it vanishes. 647. We now turn our attention to those creatures which belong to light, air and dry warmth, and it is here that we first find ourselves in the living region of colours. Here, in exquisitely organised parts, the elementary colours present themselves in their greatest purity and beauty. They indicate, however, that the creatures they adorn, are still low in the scale of organisation, precisely because these colours can thus appear, as it were, unwrought. Here, too, heat seems to contribute much to their development. 648. We find insects which may be considered altogether as concentrated colouring matter; among these, the cochineals especially are celebrated; with regard to these we observe that their mode of settling on vegetables, and even nestling in them, at the same time produces those excrescences which are so useful as mordants in fixing colours. 649. But the power of colour, accompanied by regular organisation, exhibits itself in the most striking manner in those insects which require a perfect metamorphosis for their development--in scarabæ, and especially in butterflies. 650. These last, which might be called true productions of light and air, often exhibit the most beautiful colours, even in their chrysalis state, indicating the future colours of the butterfly; a consideration which, if pursued further hereafter, must undoubtedly afford a satisfactory insight into many a secret of organised being. 651. If, again, we examine the wings of the butterfly more accurately, and in its net-like web discover the rudiments of an arm, and observe further the mode in which this, as it were, flattened arm is covered with tender plumage and constituted an organ of flying; we believe we recognise a law according to which the great variety of tints is regulated. This will be a subject for further investigation hereafter. 652. That, again, heat generally has an influence on the size of the creature, on the accomplishment of the form, and on the greater beauty of the colours, hardly needs to be remarked. LIII. BIRDS. 653. The more we approach the higher organisations, the more it becomes necessary to limit ourselves to a few passing observations; for all the natural conditions of such organised beings are the result of so many premises, that, without having at least hinted at these, our remarks would only appear daring, and at the same time insufficient. 654. We find in plants, that the consummate flower and fruit are, as it were, rooted in the stem, and that they are nourished by more perfect juices than the original roots first afforded; we remark, too, that parasitical plants which derive their support from organised structures, exhibit themselves especially endowed as to their energies and qualities. We might in some sense compare the feathers of birds with plants of this description; the feathers spring up as a last structural result from the surface of a body which has yet much in reserve for the completion of the external economy, and thus are very richly endowed organs. 655. The quills not only grow proportionally to a considerable size, but are throughout branched, by which means they properly become feathers, and many of these feathered branches are again subdivided; thus, again, recalling the structure of plants. 656. The feathers are very different in shape and size, but each still remains the same organ, forming and transforming itself according to the constitution of the part of the body from which it springs. 657. With the form, the colour also becomes changed, and a certain law regulates the general order of hues as well as that particular distribution by which a single feather becomes party coloured, It is from this that all combination of variegated plumage arises, and whence, at last, the eyes in the peacock's tail are produced. It is a result similar to that which we have already unfolded in treating of the metamorphosis of plants, and which we shall take an early opportunity to prove. 658. Although time and circumstances compel us here to pass by this organic law, yet we are bound to refer to the chemical operations which commonly exhibit themselves in the tinting of feathers in a mode now sufficiently known to us. 659. Plumage is of all colours, yet, on the whole, yellow deepening to red is commoner than blue. 660. The operation of light on the feathers and their colours, is to be remarked in all cases. Thus, for example, the feathers on the breast of certain parrots, are strictly yellow; the scale-like anterior portion, which is acted on by the light, is deepened from yellow to red. The breast of such a bird appears bright-red, but if we blow into the feathers the yellow appears. 661. The exposed portion of the feathers is in all cases very different from that which, in a quiet state, is covered; it is only the exposed portion, for instance, in ravens, which exhibits the iridescent appearance; the covered portion does not: from which indication, the feathers of the tail when ruffled together, may be at once placed in the natural order again. LIV. MAMMALIA AND HUMAN BEINGS. 662. Here the elementary colours begin to leave us altogether. We are arrived at the highest degree of the scale, and shall not dwell on its characteristics long. 663. An animal of this class is distinguished among the examples of organised being. Every thing that exhibits itself about him is living. Of the internal structure we do not speak, but confine ourselves briefly to the surface. The hairs are already distinguished from feathers, inasmuch as they belong more to the skin, inasmuch as they are simple, thread-like, not branched. They are however, like feathers, shorter, longer, softer, and firmer, colourless or coloured, and all this in conformity to laws which might be defined. 664. White and black, yellow, yellow-red and brown, alternate in various modifications, but they never appear in such a state as to remind us of the elementary hues. On the contrary, they are all broken colours subdued by organic concoction, and thus denote, more or less, the perfection of life in the being they belong to. 665. One of the most important considerations connected with morphology, so far as it relates to surfaces, is this, that even in quadrupeds the spots of the skin have a relation with the parts underneath them. Capriciously as nature here appears, on a hasty examination, to operate, she nevertheless consistently observes a secret law. The development and application of this, it is true, are reserved only for accurate and careful investigation and sincere co-operation. 666. If in some animals portions appear variegated with positive colours, this of itself shows how far such creatures are removed from a perfect organisation; for, it may be said, the nobler a creature is, the more all the mere material of which he is composed, is disguised by being wrought together; the more essentially his surface corresponds with the internal organisation, the less can it exhibit the elementary colours. Where all tends to make up a perfect whole, any detached specific developments cannot take place. 667. Of man we have little to say, for he is entirely distinct from the general physiological results of which we now treat. So much in this case is in affinity with the internal structure, that the surface can only be sparingly endowed. 668. When we consider that brutes are rather encumbered than advantageously provided with intercutaneous muscles; when we see that much that is superfluous tends to the surface, as, for instance, large ears and tails, as well as hair, manes, tufts; we see that nature, in such cases, had much to give away and to lavish. 669. On the contrary, the general surface of the human form is smooth and clean, and thus in the most perfect examples, the beautiful forms are apparent; for it may be remarked in passing, that a superfluity of hair on the chest, arms, and lower limbs, rather indicates weakness than strength. Poets only have sometimes been induced, probably by the example of the ferine nature, so strong in other respects, to extol similar attributes in their rough heroes. 670. But we have here chiefly to speak of colour, and observe that the colour of the human skin, in all its varieties, is never an elementary colour, but presents, by means of organic concoction, a highly complicated result.--Note X. 671. That the colour of the skin and hair has relation with the differences of character, is beyond question; and we are led to conjecture that the circumstance of one or other organic system predominating, produces the varieties we see. A similar hypothesis may be applied to nations, in which case it might perhaps be observed, that certain colours correspond with certain confirmations, which has always been observed of the negro physiognomy. 672. Lastly, we might here consider the problematical question, whether all human forms and hues are not equally beautiful, and whether custom and self-conceit are not the causes why one is preferred to another? We venture, however, after what has been adduced, to assert that the white man, that is, he whose surface varies from white to reddish, yellowish, brownish, in short, whose surface appears most neutral in hue and least inclines to any particular or positive colour, is the most beautiful. On the same principle a similar point of perfection in human conformation may be defined hereafter, when the question relates to form. We do not imagine that this long-disputed question is to be thus, once for all, settled, for there are persons enough who have reason to leave this significancy of the exterior in doubt; but we thus express a conclusion, derived from observation and reflection, such as might suggest itself to a mind aiming at a satisfactory decision. We subjoin a few observations connected with the elementary chemical doctrine of colours.--Note Y. LV. PHYSICAL AND CHEMICAL EFFECTS OF THE TRANSMISSION OF LIGHT THROUGH COLOURED MEDIUMS. 673. The physical and chemical effects of colourless light are known, so that it is unnecessary here to describe them at length. Colourless light exhibits itself under various conditions as exciting warmth, as imparting a luminous quality to certain bodies, as promoting oxydation and de-oxydation. In the modes and degrees of these effects many varieties take place, but no difference is found indicating a principle of contrast such as we find in the transmission of coloured light. We proceed briefly to advert to this. 674. Let the temperature of a dark room be observed by means of a very sensible air-thermometer; if the bulb is then brought to the direct sun light as it shines into the room, nothing is more natural than that the fluid should indicate a much higher degree of warmth. If upon this we interpose coloured glasses, it follows again quite naturally that the degree of warmth must be lowered; first, because the operation of the direct light is already somewhat impeded by the glass, and again, more especially, because a coloured glass, as a dark medium, admits less light through it. 675. But here a difference in the excitation of warmth exhibits itself to the attentive observer, according to the colour of the glass. The yellow and the yellow-red glasses produce a higher temperature than the blue and blue-red, the difference being considerable. 676. This experiment may be made with the prismatic spectrum. The temperature of the room being first remarked on the thermometer, the blue coloured light is made to fall on the bulb, when a somewhat higher degree of warmth is exhibited, which still increases as the other colours are gradually brought to act on the mercury. If the experiment is made with the water-prism, so that the white light can be retained in the centre, this, refracted indeed, but not yet coloured light, is the warmest; the other colours, stand in relation to each other as before. 677. As we here merely describe, without undertaking to deduce or explain this phenomenon, we only remark in passing, that the pure light is by no means abruptly and entirely at an end with the red division in the spectrum, but that a refracted light is still to be observed deviating from its course and, as it were, insinuating itself beyond the prismatic image, so that on closer examination it will hardly be found necessary to take refuge in invisible rays and their refraction. 678. The communication of light by means of coloured mediums exhibits the same difference. The light communicates itself to Bologna phosphorus through blue and violet glasses, but by no means through yellow and yellow-red glasses. It has been even remarked that the phosphori which have been rendered luminous under violet and blue glasses, become sooner extinguished when afterwards placed under yellow and yellow-red glasses than those which have been suffered to remain in a dark room without any further influence. 679. These experiments, like the foregoing, may also be made by means of the prismatic spectrum, when the same results take place. 680. To ascertain the effect of coloured light on oxydation and de-oxydation, the following means may be employed:--Let moist, perfectly white muriate of silver[1] be spread on a strip of paper; place it in the light, so that it may become to a certain degree grey, and then cut it in three portions. Of these, one may be preserved in a book, as a specimen of this state; let another be placed under a yellow-red, and the third under a blue-red glass. The last will become a darker grey, and exhibit a de-oxydation; the other, under the yellow-red glass, will, on the contrary, become a lighter grey, and thus approach nearer to the original state of more perfect oxydation. The change in both may be ascertained by a comparison with the unaltered specimen. 681. An excellent apparatus has been contrived to perform these experiments with the prismatic image. The results are analogous to those already mentioned, and we shall hereafter give the particulars, making use of the labours of an accurate observer, who has been for some time carefully prosecuting these experiments.[2] [1] Now generally called chloride of silver: the term in the original is Hornsilber.--T. [2] The individual alluded to was Seebeck: the result of his experiments was published in the second volume.--T. LVI. CHEMICAL EFFECT IN DIOPTRICAL ACHROMATISM. 682. We first invite our readers to turn to what has been before observed on this subject (285, 298), to avoid unnecessary repetition here. 683. We can thus give a glass the property of producing much wider coloured edges without refracting more strongly than before, that is, without displacing the object much more perceptibly. 684. This property is communicated to the glass by means of metallic oxydes. Minium, melted and thoroughly united with a pure glass, produces this effect, and thus flint-glass (291) is prepared with oxyde of lead. Experiments of this kind have been carried farther, and the so-called butter of antimony, which, according to a new preparation, may be exhibited as a pure fluid, has been made use of in hollow lenses and prisms, producing a very strong appearance of colour with a very moderate refraction, and presenting the effect which we have called hyperchromatism in a very vivid manner. 685. In common glass, the alkaline nature obviously preponderates, since it is chiefly composed of sand and alkaline salts; hence a series of experiments, exhibiting the relation of perfectly alkaline fluids to perfect acids, might lead to useful results. 686. For, could the maximum and minimum be found, it would be a question whether a refracting medium could not be discovered, in which the increasing and diminishing appearance of colour, (an effect almost independent of refraction,) could not be done away with altogether, while the displacement of the object would be unaltered. 687. How desirable, therefore, it would be with regard to this last point, as well as for the elucidation of the whole of this third division of our work, and, indeed, for the elucidation of the doctrine of colours generally, that those who are occupied in chemical researches, with new views ever opening to them, should take this subject in hand, pursuing into more delicate combinations what we have only roughly hinted at, and prosecuting their inquiries with reference to science as a whole.