The colouring matters and dyes, their solutions, and the substances dyed with them, are not, of course, luminous, but they do convert white light which strikes upon or traverses them into coloured light, and that is why they, in fact, appear either as coloured substances or solutions. The explanation of the coloured appearance is that the coloured substances or solutions have the power to absorb from the white light that strikes or traverses them, all the rays of the spectrum but those which are of the colour of the substance or solution in question, these latter being thrown off or reflected, and so striking the eye of the observer. Take a solution of Magenta, for example, and place a light behind it. All the rays of that white light are absorbed except the red ones, which pass through and are seen. Thus the liquid appears red. If a dyed piece be taken, the light strikes it, and if a pure red, from that light all the rays but red are absorbed, and so red light alone is reflected from its surface. But this is not all with a dyed fabric, for here the light is not simply reflected light; part of it has traversed the upper layers of that coloured body, and is then reflected from the interior, losing a portion of its coloured rays by absorption. This reflected coloured light is always mixed with a certain amount of white light reflected from the actual surface of the body before penetrating its uppermost layer. Thus, if dyed fabrics are examined by the spectroscope, the same appearances are generally observed as with the solution of the corresponding colouring matters. An absorption spectrum is in each case obtained, but the one from the solution is the purer, for it does not contain the mixed white light reflected from the surfaces of coloured objects. Let us now take an example. We will take a cylinder glass full of picric acid in water, and of a yellow colour. Now when I pass white light through that solution and examine the emerging light, which looks, to my naked eye, yellow, I find by the spectroscope that what has taken place is this: the blue part of the spectrum is totally extinguished as far as G and 2/3 of F. That is all. Then why, say you, does that liquid look yellow if all the rest of those rays pass through and enter the eye, namely, the blue-green with a trifle of blue, the green, yellow, orange, and red? The reason is this: we have already seen that the colours complementary to, and so producing white light with red, are green and greenish-blue or bluish-green. Hence these cancel, so to say, and we only see yellow. We do not see a pure yellow, then, in picric acid, but yellow with a considerable amount of white. Here is a piece of scarlet paper. Why does it appear scarlet? Because from the white light falling upon it, it practically absorbs all the rays of the spectrum except the red and orange ones, and these it reflects. If this be so, then, and we take our spectrum band of perfectly pure colours and pass our strip of scarlet paper along that variously coloured band of light, we shall be able to test the truth of several statements I have made as to the nature of colour. I have said colour is only an impression, and not a reality; and that it does not exist apart from light. Now, I can show you more, namely, that the colour of the so-called coloured object is entirely dependent on the existence in the light of the special coloured rays which it radiates, and that this scarlet paper depends on the red light of the spectrum for the existence of its redness. On passing the piece of scarlet paper along the coloured band of light, it appears red only when in the red portion of the spectrum, whilst in the other portions, though it is illumined, yet it has no colour, in fact it looks black. Hence what I have said is true, and, moreover, that red paper looks red because, as you see, it absorbs and extinguishes all the rays of the spectrum but the red ones, and these it radiates. A bright green strip of paper placed in the red has no colour, and looks black, but transferred to the pure green portion it radiates that at once, does not absorb it as it did the red, and so the green shines out finely. I have told you that sodium salts give to a colourless flame a fine monochromatic or pure yellow colour. Now, if this be so, and if all the light available in this world were of such a character, then such a colour as blue would be unknown. We will now ask ourselves another question, "We have a new blue colouring matter, and we desire to know if we may expect it to be one of the greatest possible brilliancy, what spectroscopic conditions ought it to fulfil?" On examining a solution of it, or rather the light passing through a solution of it, with the spectroscope, we ought to find that all the rays of the spectrum lying between and nearly to H and b (Fig. 16), i.e. all the bluish-violet, blue, and blue-green rays pass through it unchanged, unabsorbed, whilst all the rest should be completely absorbed. In like manner a pure yellow colour would allow all the rays lying between orange-red and greenish-yellow (Fig. 16) to pass through unchanged, but would absorb all the other colours of the spectrum.
Now we come to the, for you, most-important subject of mixtures of colours and their effects. Let us take the popular case of blue and yellow producing green. We have seen that the subjective effect of the mixture of blue and yellow light on the eye is for the latter to lose sense of colour, since colour disappears, and we get what we term white light; in strict analogy to this the objective effect of a pure yellow pigment and a blue is also to destroy colour, and so no colour comes from the object to the eye; that object appears black. Now the pure blue colouring matter would not yield a green with the pure yellow colouring matter, for if you plot off the two absorption spectra as previously described, on to the spectrum (Fig. 16), you will find that all the rays would be absorbed by the mixture, and the result would be a black. But, now, suppose a little less pure yellow were taken, one containing a little greenish-yellow and a trifle of green, and also a little orange-red on the other side to red, then whereas to the eye that yellow might be as good as the first; now, when mixed with a blue, we get a very respectable green. But, and this is very important, although of the most brilliant dyes and colours there are probably no two of these that would so unite to block out all the rays and produce black, yet this result can easily and practically be arrived at by using three colouring matters, which must be as different as possible from one another. Thus a combination of a red, a yellow, and a blue colouring matter, when concentrated enough, will not let any light pass through it, and can thus be used for the production of blacks, and this property is made use of in dyeing. And now we see why a little yellow dye is added to our coal-tar black. A purplish shade would else be produced; the yellow used is a colour complementary to that purple, and it absorbs just those blue and purple rays of the spectrum necessary to illuminate by radiation that purple, and vice versâ; both yellow and purple therefore disappear. In like manner, had the black been of a greenish shade, I should have added Croceine Orange, which on the fabric would absorb just those green and bluish rays of light necessary to radiate from and illumine that greenish part, and the greenish part would do the like by the orange rays; the effects would be neutralised, and all would fall together into black.
THE END.
INDEX
Acetone, [64]
Acid, boric. See Boric acid.
" carbolic. See Phenol.
" colours, mordanting, [74]
" hydrochloric. See Hydrochloric acid.
" nitric. See Nitric acid.
" sulphuric. See Sulphuric acid.
Acids, distinguishing, from alkalis, [23], [49]
" neutralisation of, [50]
" properties of, [49]
" specific gravities of, [49]
Affinity, chemical, [71]
Alizarin, [75], [76], [80], [83], [91], [99]
" blue, [90]
" paste, [91]
" pure, [91]
" purple, [77]
" red, [77]
Alkali, manufacture of, by ammonia-soda process, [55]
" manufacture of, by electrolytic process, [56]
" manufacture of, by Leblanc process, [53]
Alkalis, distinguishing, from acids, [23], [49]
" neutralisation of, [50]
" properties of, [49]
" specific gravities of, [49]
Alum, cake, [73]
Aluminium sulphate, [73]
Ammonia, [23], [95]
Ammonia-soda process, [55]
Aniline, [91]
" black, [81]
" constitution of, [96]
" preparation of, [96]
" reaction of [97]
" violet [77], [81]
Animal fibres. See Fibres.
Annatto, [83], [85], [87]
Anthracene, [90]
Archil. See Orchil.
Aurin, [91], [98]
Azo dyestuffs, [98]
Barwood, [99]
Basic colours or dyestuffs, mordanting, [76]
Bast fibres. See Fibres.
Bastose, [4]
Bastose, distinction between, and cellulose, [4]
Beaumé hydrometer degrees, [31]
Benzene, [90], [96]
Bixin, [88]
Black-ash process, [54]
Blue colour, absorption spectrum of pure, [114]
Boilers, incrustations in, [42]
Boiling-point, effect of pressure on, [32]
" of water, effect of dissolved salts on, [36]
" of water, effect of increase of pressure on, [35]
Borax, [59]
" tests of purity of, [59]
Boric acid, [57]
Boronitrocalcite, [59]
Brasilin, [99]
Brazil wood, [99]
Camwood, [99]
Carbolic acid. See Phenol.
Carminic acid, [76]
Carré ice-making machine, [32]
Carrotting. See Sécretage.
Carthamic acid, [87]
Carthamin, [87]
Cellulose, action of cupric-ammonium solutions on, [5]
" composition of, [3]
" distinction between, and bastose, [4]
" properties of pure, [5]
Cholesterol, [100]
Chrome mordanting, [78]
Chrome orange, [84]
" yellow, [84]
Chroming, over-, [78]
Clark's soap test, [43]
Coal-tar, [90]
" yield of valuable products from, [90]
Cochineal, [75], [76], [82], [83], [99]
Cœrulein, [90]
Colour, absorption spectrum of pure blue, [114]
" absorption spectrum of pure yellow, [114]
" acids, [77]
" bases, [77]
" nature of, [107]
Coloured substances, spectra of, [112]
Colours, acid, mordanting of, [74]
" basic, [75]
" classification of, [79]
" complementary, [109]
" mixed, spectra of, [115]
" pigment, [110]
" primary, [110]
" spectral, [110]
Conditioning establishments, [21]
Congo red, [71]
Copper salts, dissolving, in iron pans, [39]
" wet method of extracting, [38]
Corrosion caused by fatty acids, [35]
Cotton and woollen goods, separation of mixed, [5]
Cotton fibre, action of basic zinc chloride on, [5]
" composition of, [3]
" dimensions of, [2]
" stomata in cuticle of, [2]
" structure of, [1]
Cotton-silk fibre, [3]
" " composition of, [3]
Crookes' heat-indicating paint, [107]
Cudbear, [86]
Cupric ammonium solution, action of, on cellulose, [5]
Curcumin, [87]
Dextrin, [4]
Dyeing felt hats deep black, [106]
" " effect of stiffening and proofing process in, [65], [103]
" of wool and felt with coal-tar colours, [105]
" of wool and fur, [100]
" power of coal-tar dyestuffs, [93]
" with mixed coal-tar colours, [106]
Dyestuffs, adjectiv, [83], [99]
" azo, [98]
" classification of, [79]
" coal-tar, [90]
" " dyeing power of, [93]
" " yield of, [91]
" mineral, [83]
" monogenetic, [81]
" pigment, [83]
" polygenetic, [82]
" substantive, [83]
" " artificial, [89]
" " natural, [85]
Equivalence, law of, [49]
Fats, decomposition of, by superheated steam, [35]
Felt, dyeing, deep black, [106]
" " with coal-tar colours, [105]
Felting, dilute acid for promoting, [22]
" effect of water in, [21]
" fur, [15]
" interlocking of scales in, [13]
" preparation of fur for, [18]
" unsuitability of dead wool for, [18]
Fibre, cotton. See Cotton.
" cotton-silk. See Cotton-silk.
" flax. See Flax.
" jute. See Jute.
" silk. See Silk.
" wool. See Wool.
Fibres, action of acids on textile, [5]
" " alkaline solution of copper and glycerin on textile, [28]
" " alkalis on textile, [5]
" " caustic soda on textile , [28]
" " copper-oxide-ammonia on textile, [28]
" " nitric acid on textile, [28]
" " steam on textile, [5]
" " sulphuric acid on textile, [27]
Fibres, animal, [6]
" bast, [3]
" vegetable, [1]
" " and animal, determining, in mixture, [27]
" " and animal, distinguishing, [4], [5]
" " and animal, distinguishing and separating, [24]
Fibroïn, [7]
Flax fibre, action of basic zinc chloride on, [5]
" composition of, [3]
" structure of, [2]
Fraunhofer's lines, [111], [112]
Fur, [8]
" action of acids on, [23]
" " of alkalis on, [24]
" " on, in sécretage process, [17]
" chrome mordanting of, [77]
" composition of, [22]
" felting, [15]
" finish and strength of felted, effect of boiling water on, [22]
" hygroscopicity of, [20]
" preparation of, for felting, [18]
" sécretage or carrotting of, [17]
" stiffening and proofing of felted, [66]
" sulphur in, reagents for detection of, [26]
Fustic, [99]
Gallein, [82], [83]
Gallnuts, [99]
Garancine, [99]
Guy-Lussac tower, [52]
Glover tower, [52]
Glucose, [4]
Greening of black hats, [65]
Hæmatein, [76], [78] 83, [99]
Hair, [8]
" cells from, [11]
" distinction between, and wool, [12], [14]
" dyeing, [26]
" growth of, [8]
" scales from, [11]
" " of, action of reagents on, [12]
" scaly structure of, [11]
" structure of, [8], [9]
" sulphur in, reagents for detection of, [26]
Hargreaves & Robinson's process, [53]
Hats dyed logwood black, deterioration of, [104]
" greening of black, [65]
" stiffening and proofing of, [63], [64]
" stiffening and proofing of, by Cheetham's process, [66]
" stiffening and proofing of, by Continental process, [66]
" stiffening and proofing process, effect of, in dyeing, [65], [103]
Heat, latent, [32], [33]
" " of steam, [34]
" " of water, [34]
Heddebault's process of separating mixed cotton and woollen goods, [5]
Hydrochloric acid, manufacture of, by Hargreaves & Robinson's process, [53]
" " manufacture of, by salt-cake process, [53]
Ice, heat of liquefaction of, [34]
Ice-making machine, Carré, [32]
Indican, [85]
Indicators, [50], [70]
Indigo, [85]
" artificial, [86]
" blue, [85]
" recovery of, from indigo-dyed woollen goods, [24]
" vat, [86]
" white, [85]
Insoluble compounds, precipitation of, from solutions, [38]
Iron liquor. See Mordant, iron.
Jute fibre, [3]
" composition of, [4]
Lac, button, [63]
" dye, [62], [99]
" seed, [62]
" stick, [62]
See also Shellac.
Lakes, colour, [75]
Latent heat. See Heat.
Leblanc process, [53]
Light, analysis of white, [107]
" composition of white, [107]
" homogeneous or monochromatic, [108], [110]
" rays, refraction of, [108]
Linen fibre. See Flax.
Litmus, [70], [86]
Logwood, [75], [76], [78], [83], [99]
Logwood black, [78], [81], [104]
" " deterioration of hats dyed with, [104]
Madder, [80], [83], [99]
Magenta, [76], [80], [83], [91], [97]
Marsh gas, [95]
Mercuric nitrate, use of, for the sécretage of fur, [17]
Merino wool, [15]
Methane. See Marsh gas.
Methyl alcohol. See Wood spirit.
" green, [97]
" violet, [97]
Mirbane, essence of, [96]
Molisch's test, [4]
Mordant, alumina, [64], [75]
" antimony, [76]
" iron, [64], [76]
" tannin, [76]
" tin, [76]
Mordanting acid (phenolic) colours, [74]
" basic colours, [76]
" chrome, [77]
" woollen fabrics, [75]
Mordants, [69]
" fatty acid, [77]
Naphthalene, [90], [98]
Naphthol yellow, [91]
Naphthols, [91], [98]
Naphthylamine, [91]
Nitric acid, [95]
" manufacture of, [52]
Nitrobenzene, [96]
Nitroprusside of soda, [26]
Oils, decomposition of, by superheated steam, [35]
Orcèin, [86]
Orchil, [85], [86]
Orcin, [86]
Orellin, [88]
Over-chroming, See Chroming.
Paint, Crookes' heat-indicating, [107]
Persian berries, [75], [99]
Phenol, [90]
" constitution of, [98]
Phenolic colours. See Acid colours.
Phenolphthalein, [70]
Picric acid, [81], [91]
" absorption spectrum of, [113]
" constitution of, [98]
Plumbate of soda, [26]
Potassium, decomposition of water by, [25], [30]
Proofing mixture, [63]
" process, [64]
" " Cheetham's, [66]
" " Continental, [66]
" " effect of, in dyeing, [65], [103]
Purpurin, [99]
Quercitron, [99]
Red liquor. See Mordant, alumina.
Refraction of light rays, [108]
Safflower, [85], [87]
Salt-cake process, [53]
Salts, [49]
" acid, [70], [71]
" basic, [71]
" neutral or normal, [71]
" stable, [72]
" unstable, [72]
Santalin, [99]
Santalwood, [99]
Sealing-wax, coloured, [103]
Sécretage of fur, [17]
" process, injury to fur in, [17]
Sericin, [7]
Shellac, [62]
" colouring of, [103]
" rosin in, detection of, [63]
" solvents for, [63]
See also Lac.
Silk fibre, action of acids on, [7]
" " " of alkaline solution of, copper and glycerin on, [7]
" " " of alkalis on, [7]
" " " of basic zinc chloride on, [7]
" " bleaching of, [7]
" " composition of, [7]
" " structure of, [6]
" " ungumming of, [7]
" glue, [7]
" gum, [7]
Soap, [60]
" alkali in, detection of, [61]
" oleic acid, [101]
" palm oil, [101]
" water in, determination of, [60]
Soda. See Alkali.
Solution, [36]
" precipitation of insoluble compounds from, [38]
Specific gravity, [30]
Spectra of coloured substances [112]
Spectroscope, [111]
Spectrum, [108]
" absorption, [113]
" continuous, [111]
" discontinuous or line, [111]
Spirits of salt. See Hydrochloric acid.
Starch, [4]
Steam, [31]
" latent heat of, [34]
Stiffening mixture, [63]
" process, [64]
" " Cheetham's, [66]
" " Continental, [66]
" " effect of, in dyeing [65], [103]
Suint. See Wool grease.
Sulphur in wool, fur, and hair, reagents for detection of, [26]
Sulphuric acid, manufacture of, [50]
" " " by contact process, [52]
" " " by lead chamber process, [51]
Sumach, [99]
Tannins, [99]
Tincal, [59]
Tiza, [59]
Toluene, [90]
Toluidine, [91]
Turmeric, [80], [83], [85], [87]
Twaddell hydrometer degrees, [31]
Ultramarine blue, [81]
Ultramarine green, [81]
" rose-coloured, [81]
Valency, [71]
Vegetable fibres. See Fibres.
Veneering process, [66]
Vermilline scarlet, [91]
Vitriol. See Sulphuric acid.
Water, [29]
" boiling of [31]
" boiling-point of, effect of dissolved salts on [36]
" boiling-point of, effect of increase of pressure on, [35]
" chlorides in, detection of, [47]
" composition of, [29]
" contamination of, by factories, [45]
" copper in, detection of, [46]
" decomposition of, by potassium, [25], [30]
" filtration of, [47]
" hard, [41], [42]
" " Clark's soap test for, [43]
" " softening of, [41]
" " waste of soap by, [43]
" hardness, temporary and permanent, of, [42]
" impurities in, [42]
" " effect of, in dyeing, [42]
" " ferruginous, [44]
" iron in, detection of, [46]
" latent heat of, [34]
" lead in, detection of, [47]
" lime in, detection of, [46]
" magnesium in, detection of, [46]
" purification of, [45]
" purity of, tests for, [46]
" soft, [40]
" effect of carbonic acid in hardening, [40]
" sulphates in, detection of, [24]
Wood acid, [64]
" destructive distillation of, [64]
" spirit, [64]
Wool, chrome mordanting of, [77]
" dead: why it will not felt, [18]
" dyeing, with coal-tar colours, [105]
" felted, effect of boiling water on finish and strength of, [22]
" felted, effect of stiffening process on finish of, [66], [103]
" felting of, interlocking of scales in, [13]
" fibre, [8]
" " action of acids on, [23]
" " " of alkalis on, [24]
" " composition of, [22]
" " curly structure of, [15]
" " distinction between, and hair, [12], [14]
" " growth of, [8]
" " hygroscopicity of, [20]
" " structure of, from diseased sheep, [19]
" " sulphur in, reagents for detection of, [26]
" grease, [100]
" kempy, [19]
" merino, [15]
" mordanting, [75]
" scouring, [101]
" stripping of, [23]
Woollen goods, indigo-dyed, recovery of indigo from, [24]
" " mixed cotton and, separation of, [5]
Xylenes, [90]
Yellow colour, absorption spectrum of pure, [114]
Yolk. See Wool grease.