Chapter 4

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of | of |= 12 |012 lb. | |alpha- | | | lb. of | Aniline |Toluidine|lb. of|of Xylene | |Naphthylamine | | |Alizarin| | |Aurin.|= 007 lb.| |= 711 lb. of | | | (20%). | | | |of | |Vermilline | | | | ________________/ | |Xylidine | |Scarlet | | | | = 0623 lb of | | | |RRR; or 475 | | | | Magenta. | | | |lb. of | | | | | | | | |alpha- | | | | or 110 | | | | |or beta- | | | | lb. of | | | | |Naphthol | | | | Aniline | | | | |= 950 lb. of | | | | yields | | | | |Naphthol | | | | 123 lb. | | | | |Yellow | | | | of Methyl| | | | | | | | | Violet. | | | | | | | | | ---------+---------+------+----------+----+--------------+---+---+--------+----

[Footnote 2: This table was compiled by Mr. Ivan Levinstein, of Manchester.]

The next table (see Table B) shows you the dyeing power of the colouring matters derived from 1 ton of Lancashire coal, which will astonish any thoughtful mind, for the Magenta will dye 500 yards of flannel, the Aurin 120 yards, the Vermilline Scarlet 2560 yards, and the Alizarin 255 yards (Turkey-red cotton cloth).

The next table (Table C) shows the latent dyeing power resident, so to speak, in 1 lb. of coal.

By a very simple experiment a little of a very fine violet dye can be made from mere traces of the materials. One of the raw materials for preparing this violet dye is a substance with a long name, which itself was prepared from aniline. This substance is tetramethyldiamidobenzophenone, and a little bit of it is placed in a small gla.s.s test-tube, just moistened with a couple of drops of another aniline derivative called dimethylaniline, and then two drops of a fuming liquid, trichloride of phosphorus, added. On simply warming this mixture, the violet dyestuff is produced in about a minute. Two drops of the mixture will colour a large cylinder of water a beautiful violet.

The remainder (perhaps two drops more) will dye a skein of silk a bright full shade of violet. Here, then, is a magnificent example of enormous tinctorial power. I must now draw the rein, or I shall simply transport you through a perfect wonderland of magic, bright colours and apparent chemical conjuring, without, however, an adequate return of solid instruction that you can carry usefully with you into every-day life and practice.

TABLE B.[3]

------------------------------------------------------------------------------- DYEING POWERS OF COLOURS FROM 1 TON OF LANCASHIRE COAL.

|violet. |yellow. | | | ------------+------------+------------+-------------+-------------+------------

TABLE C.[3]

------------------------------------------------------------------------------- DYEING POWERS OF COLOURS FROM 1 LB. OF LANCASHIRE COAL.

[Footnote 3: These tables were compiled by Mr. Ivan Levinstein, of Manchester.]

Before we go another step, I must ask and answer, therefore, a few questions. Can we not get some little insight into the structure and general mode of developing the leading coal-tar colours which serve as types of whole series? I will try what can be done with the little knowledge of chemistry we have so far acc.u.mulated. In our earlier lectures we have learnt that water is a compound of hydrogen and oxygen, and in its compound atom or molecule we have two atoms of hydrogen combined with one of oxygen, symbolised as H_{2}O. We also learnt that ammonia, or spirits of hartshorn, is a compound of hydrogen with nitrogen, three atoms of hydrogen being combined with one of nitrogen, thus, NH_{3}. An example of a hydrocarbon or compound of carbon and hydrogen, is marsh gas (methane) or firedamp, CH_{4}. Nitric acid, or _aqua fortis_, is a compound of nitrogen, oxygen, and hydrogen, one atom of the first to three of the second and one of the third--NO_{3}H. But this nitric acid question forces me on to a further statement, namely, we have in this formula or symbol, NO_{3}H, a twofold idea--first, that of the compound as a whole, an acid; and secondly, that it is formed from a substance without acid properties by the addition of water, H_{2}O, or, if we like, HOH. This substance contains the root or radical of the nitric acid, and is NO_{2}, which has the power of replacing one of the hydrogen atoms, or H, of water, and so we get, instead of HOH, NO_{2}OH, which is nitric acid. This is chemical replacement, and on such replacement depends our powers of building up not only colours, but many other useful and ornamental chemical structures. You have all heard the old-fashioned statement that "Nature abhors a vacuum." We had a very practical example of this when in our first lecture on water I brought an electric spark in contact with a mixture of free oxygen and hydrogen in a gla.s.s bulb. These gases at once united, three volumes of them condensing to two volumes, and these again to a minute particle of liquid water. A vacuum was left in that delicate gla.s.s bulb whilst the pressure of the atmosphere was crushing with a force of 15 lb. on the square inch on the outside of the bulb, and thus a violent crash was the result of Nature"s abhorrence. There is such a kind of thing, though, and of a more subtle sort, which we might term a chemical vacuum, and it is the result of what we call chemical valency, which again might be defined as the specific chemical appet.i.te of each substance.

Let us now take the case of the production of an aniline colour, and let us try to discover what aniline is, and how formed. I pointed to benzene or benzol in the table as a hydrocarbon, C_{6}H_{6}, which forms a princ.i.p.al colour-producing const.i.tuent of coal-tar. If you desire to produce chemical appet.i.te in benzene, you must rob it of some of its hydrogen. Thus C_{6}H_{5} is a group that would exist only for a moment, since it has a great appet.i.te for H, and we may say this appet.i.te would go the length of at once absorbing either one atom of H (hydrogen) or of some similar substance or group having a similar appet.i.te. Suppose, now, I place some benzene, C_{6}H_{6}, in a flask, and add some nitric acid, which, as we said, is NO_{2}OH. On warming the mixture we may say a tendency springs up in that OH of the nitric acid to effect union with an H of the C_{6}H_{6} (benzene) to form HOH (water), when an appet.i.te is at once left to the remainder, C_{6}H_{5}--on the one hand, and the NO_{2}--on the other, satisfied by immediate union of these residues to form a substance C_{6}H_{6}NO_{2}, nitro-benzene or "essence of mirbane," smelling like bitter almonds. This is the first step in the formation of aniline.

I think I have told you that if we treat zinc sc.r.a.ps with water and vitriol, or water with pota.s.sium, we can rob that water of its oxygen and set free the hydrogen. It is, however, a singular fact that if we liberate a quant.i.ty of fresh hydrogen amongst our nitrobenzene C_{6}H_{5}NO_{2}, that hydrogen tends to combine, or evinces an ungovernable appet.i.te for the O_{2} of that NO_{2} group, the tendency being again to form water H_{2}O. This, of course, leaves the residual C_{6}H_{5}N: group with an appet.i.te, and only the excess of hydrogen present to satisfy it. Accordingly hydrogen is taken up, and we get C_{6}H_{5}NH_{2} formed, which is aniline. I told you that ammonia is NH_{3}, and now in aniline we find an ammonia derivative, one atom of hydrogen (H) being replaced by the group C_{6}H_{5}. I will now describe the method of preparation of a small quant.i.ty of aniline, in order to ill.u.s.trate what I have tried to explain in theory. Benzene from coal-tar is warmed with nitric acid in a flask. A strong action sets in, and on adding water, the nitrobenzene settles down as a heavy oil, and the acid water can be decanted off. After washing by decantation with water once or twice, and shaking with some powdered marble to neutralise excess of acid, the nitrobenzene is brought into contact with fresh hydrogen gas by placing amongst it, instead of zinc, some tin, and instead of vitriol, some hydrochloric acid (spirits of salt). To show you that aniline is formed, I will now produce a violet colour with it, which only aniline will give. This violet colour is produced by adding a very small quant.i.ty of the aniline, together with some bleaching powder, to a mixture of chalk and water, the chalk being added for the purpose of destroying acidity. This aniline, C_{6}H_{5}NH_{2}, is a base, and forms the foundation of all the so-called basic aniline colours. If I have made myself clear so far, I shall be contented. It only remains to be said that for making Magenta, pure aniline will not do, what is used being a mixture of aniline, with an aniline a step higher, prepared from toluene. If I were to give you the formula of Magenta you would be astonished at its complexity and size, but I think now you will see that it is really built up of aniline derivatives. Methyl Violet is a colour we have already referred to, and its chemical structure is still more complex, but it also is built up of aniline materials, and so is a basic aniline colour. Now it is possible for the colour-maker to prepare a very fine green dye from this beautiful violet (Methyl Violet). In fact he may convert the violet into the green colour by heating the first under pressure with a gas called methyl chloride (CH_{3}Cl). Methyl Violet is constructed of aniline or subst.i.tuted aniline groups; the addition of CH_{3}Cl, then, gives us the Methyl Green. But one of the misfortunes of Methyl Green is that if the fabric dyed with it be boiled with water, at that temperature (212 F.) the colour is decomposed and injured, for some of the methyl chloride in the compound is driven off.

In fact, by stronger heating we may drive off all the methyl chloride and get the original Methyl Violet back again.

But we have coal-tar colours which are not basic, but rather of the nature of acid,--a better term would be _phenolic_, or of the nature of phenol or carbolic acid. Let us see what phenol or carbolic acid is. We saw that water may be formulated HOH, and that benzene is C_{6}H_{6}.

Well, carbolic acid or phenol is a derivative of water, or a derivative of benzene, just as you like, and it is formulated C_{6}H_{5}OH. You can easily prove this by dropping carbolic acid or phenol down a red-hot tube filled with iron-borings. The oxygen is taken up by the iron to give oxide of iron, and benzene is obtained, thus: C_{6}H_{5}OH gives O and C_{6}H_{6}. But there is another hydrocarbon called naphthalene, C_{10}H_{8}, and this forms not one, but two phenols. As the name of the hydrocarbon is naphthalene, however, we call these compounds naphthols, and one is distinguished as alpha- the other as beta-naphthol, both of them having the formula C_{10}H_{7}OH. But now with respect to the colours. If we treat phenol with nitric acid under proper conditions, we get a yellow dye called picric acid, which is trinitro-phenol C_{6}H_{2}(NO_{2})_{3}OH; you see this is no aniline dye; it is not a basic colour, for it would saturate, _i.e._ destroy the basicity of bases. Again, by oxidising phenol with oxalic acid and vitriol, we get a colour dyeing silk orange, namely, Aurin, HO.C[C_{6}H_{4}(OH)]_{3}. This is also an acid or phenolic dye, as a glance at its formula will show you. Its compound atom bristles, so to say, with phenol-residues, as some of the aniline dyes do with aniline residue-groups.

We come now to a peculiar but immensely important group of colours known as the azo-dyes, and these can be basic or acid, or of mixed kind. Just suppose two ammonia groups, NH_{3} and NH_{3}. If we rob those nitrogen atoms of their hydrogen atoms, we should leave two unsatisfied nitrogen atoms, atoms with an exceedingly keen appet.i.te represented in terms of hydrogen atoms as N*** and N***. We might suppose a group, though of two N atoms partially satisfied by partial union with each other, thus--N:N--.

Now this group forms the nucleus of the azo-colours, and if we satisfy a nitrogen at one side with an aniline, and at the other with a phenol, or at both ends with anilines, and so on, we get azo-dyes produced. The number of coal-tar colours is thus very great, and the variety also.

_Adjective Colours._--As regards the artificial coal-tar adjective dyestuffs, the princ.i.p.al are Alizarin and Purpurin. These are now almost entirely prepared from coal-tar anthracene, and madder and garancine are almost things of the past. Vegetable adjective colours are Brazil wood, containing the dye-generating principle Brasilin, logwood, containing Haematein, and santal-wood, camwood, and barwood, containing Santalin.

Animal adjective colours are cochineal and lac dye. Then of wood colours we have further: quercitron, Persian berries, fustic and the tannins or tannic acids, comprising extracts, barks, fruits, and gallnuts, with also leaves and twigs, as with sumac. All these colours dye only with mordants, mostly forming with certain metallic oxides or basic salts, brightly-coloured compounds on the tissues to which they are applied.

LECTURE XI

DYEING OF WOOL AND FUR; AND OPTICAL PROPERTIES OF COLOURS

You have no doubt a tolerably vivid recollection of the ill.u.s.trations given in Lecture I., showing the structure of the fibre of wool and fur.

We saw that the wool fibre, of which fur might be considered a coa.r.s.er quality, possesses a peculiar, complex, scaly structure, the joints reminding one of the appearance of plants of the _Equisetum_ family, whilst the scaled structure resembles that of the skin of the serpent.

Now you may easily understand that a structure like this, if it is to be completely and uniformly permeated by a dye liquor or any other aqueous solution, must have those scales not only well opened, but well cleansed, because if choked with greasy or other foreign matter impervious to or resisting water, there can be no chance of the mordanting or dye liquids penetrating uniformly; the resulting dye must be of a patchy nature. All wool, in its natural state, contains a certain amount of a peculiar compound almost like a potash soap, a kind of soft soap, but it also contains besides, a kind of fatty substance united with lime, and of a more insoluble nature than the first. This natural greasy matter is termed "yolk" or "suint"; and it ought never to be thrown away, as it sometimes is by the wool-scourers in this country, for it contains a substance resembling a fat named _cholesterin_ or _cholesterol_, which is of great therapeutical value. Water alone will wash out a considerable amount of this greasy matter, forming a kind of lather with it, but not all. As is almost invariably the case, after death, the matters and secretions which in life favour the growth and development of the parts, then commence to do the opposite. It is as if the timepiece not merely comes to a standstill, but commences to run backwards. This natural grease, if it be allowed to stand in contact with the wool for some time after shearing, instead of nourishing and preserving the fibres as it does on the living animal, commences to ferment, and injures them by making them hard and brittle. We see, then, the importance of "scouring" wool for the removal of "yolk," as it is called, dirt, oil, etc. If this important operation were omitted, or incompletely carried out, each fibre would be more or less covered or varnished with greasy matter, resisting the absorption and fixing of mordant and dye. As scouring agents, ammonia, carbonate of ammonia, carbonate of soda completely free from caustic, and potash or soda soaps, especially palm-oil soaps, which need not be made with bleached palm oil, but which must be quite free from free alkali, may be used. In making these palm-oil soaps it is better to err on the side of a little excess of free oil or fat, but if more than 1 per cent. of free fat be present, lathering qualities are then interfered with. Oleic acid soaps are excellent, but are rather expensive for wool; they are generally used for silks. Either as a skin soap or a soap for scouring wools, I should prefer one containing about 1/2 per cent. of free fatty matter, of course perfectly equally distributed, and not due to irregular saponification. On the average the soap solution for scouring wool may contain about 6-1/2 oz. of soap to the gallon of water. In order to increase the cleansing powers of the soap solution, some ammonia may be added to it. However, if soap is used for wool-scouring, one thing must be borne in mind, namely, that the water used must not be hard, for if insoluble lime and magnesia soaps are formed and precipitated on the fibre, the scouring will have removed one evil, but replaced it by another. The princ.i.p.al scouring material used is one of the various forms of commercial carbonate of soda, either alone or in conjunction with soap. Whatever be the form or name under which the carbonate of soda is sold, it must be free from hydrate of soda, _i.e._ caustic soda, or, as it is also termed, "causticity." By using this carbonate of soda you may dispense with soap, and so be able, even with a hard or calcareous water, to do your wool-scouring without anything like the ill effects that follow the use of soap and calcareous water. The carbonate of soda solutions ought not to exceed the specific gravity of 1 to 2 Twaddell (1-1/2 to 3 oz. avoird. per gallon of water). The safest plan is to work with as considerable a degree of dilution and as low a temperature as are consistent with fetching the dirt and grease off. The scouring of loose wool, as we may now readily discern, divides itself into three stages: 1st, the stage in which those "yolk" or "suint"

const.i.tuents soluble in water, are removed by steeping and washing in water. This operation is generally carried out by the wool-grower himself, for he desires to sell wool, and not wool plus "yolk" or "suint," and thus he saves himself considerable cost in transport. The water used in this process should not be at a higher temperature than 113 F., and the apparatus ought to be provided with an agitator; 2nd, the cleansing or scouring proper, with a weak alkaline solution; 3rd, the rinsing or final washing in water.

Thus far we have proceeded along the same lines as the woollen manufacturer, but now we must deviate from that course, for he requires softness and delicacy for special purposes, for spinning and weaving, etc.; but the felt manufacturer, and especially the manufacturer of felt for felt hats, requires to sacrifice some of this softness and delicacy in favour of greater felting powers, which can only be obtained by raising the scales of the fibres by means of a suitable process, such as treatment with acids. This process is one which is by no means unfavourable to the dyeing capacities of the wool; on the whole it is decidedly favourable.

So far everything in the treatment of the wool has been perfectly favourable for the subsequent operations of the felt-hat dyer, but now I come to a process which I consider I should be perfectly unwarranted in pa.s.sing over before proceeding to the dyeing processes. In fact, were it not for this "proofing process" (see Lecture VII.) the dyeing of felt hats would be as simple and easy of attainment as the ordinary dyeing of whole-wool fabrics. Instead of this, however, I consider the hat manufacturer, as regards his dyeing processes as applied to the stiffer cla.s.ses of felt hats, has difficulties to contend with fully comparable with those which present themselves to the dyer of mixed cotton and woollen or Bradford goods. You have heard that the purpose of the wool-scourer is to remove the dirt, grease, and so-called yolk, filling the pores and varnishing the fibres. Now the effect of the work of the felt or felt-hat proofer is to undo nearly all this for the sake of rendering the felt waterproof and stiff. The material used, also, is even more impervious and resisting to the action of aqueous solutions of dyes and mordants than the raw wool would be. In short, it is impossible to mordant and to dye sh.e.l.lac by any process that will dye wool. To give you an idea of what it is necessary to do in order to colour or dye sh.e.l.lac, take the case of coloured sealing-wax, which is mainly composed of sh.e.l.lac, four parts, and Venice turpentine, one part. To make red sealing-wax this mixture is melted, and three parts of vermilion, an insoluble metallic pigment, are stirred in. If black sealing-wax is required, lamp-black or ivory-black is stirred in. The fused material is then cast in moulds, from which the sticks are removed on cooling. That is how sh.e.l.lac may be coloured as sealing-wax, but it is a totally different method from that by which wool is dyed. The difficulty then is this--in proofing, your hat-forms are rendered impervious to the dye solutions of your dye-baths, all except a thin superficial layer, which then has to be rubbed down, polished, and finished. Thus in a short time, since the bulk of that superficially dyed wool or fur on the top of every hat is but small, and has been much reduced by polishing and rubbing, you soon hear of an appearance of bareness--I was going to say threadbareness--making itself manifest. This is simply because the colour or dye only penetrates a very little way down into the substance of the felt, until, in fact, it meets the proofing, which, being as it ought to be, a waterproofing, cannot be dyed. It cannot be dyed either by English or German methods; neither logwood black nor coal-tar blacks can make any really good impression on it. Cases have often been described to me ill.u.s.trating the difficulty in preventing hats which have been dyed black with logwood, and which are at first a handsome deep black, becoming rather too soon of a rusty or brownish shade. Now my belief is that two causes may be found for this deterioration. One is the unscientific method adopted in many works of using the same bath practically for about a month together without complete renewal. During this time a large quant.i.ty of a muddy precipitate acc.u.mulates, rich in hydrated oxide of iron or basic iron salts of an insoluble kind. This mud amounts to no less than 25 per cent. of the weight of the copperas used. From time to time carbonate of ammonia is added to the bath, as it is said to throw up "dirt." The stuff or "dirt," chiefly an ochre-like ma.s.s stained black with the dye, and rich in iron and carbonate of iron, is skimmed off, and fresh verdigris and copperas added with another lot of hat-forms. No doubt on adding fresh copperas further precipitation of iron will take place, and so this ochre-like precipitate will acc.u.mulate, and will eventually come upon the hats like a kind of thin black mud. Now the effect of this will be that the dyestuff, partly in the fibre as a proper dye, and not a little on the fibre as if "smudged" on or painted on, will, on exposure to the weather, moisture, air, and so on, gradually oxidise, the great preponderance of iron on the fibre changing to a kind of iron-rust, corroding the fibres in the process, and thus at once accounting for the change to the ugly brownish shade, and to the rubbing off and rapid wearing away of the already too thin superficial coating of dyed felt fibre. In the final spells of dyeing in the dye-beck already referred to, tolerably thick with black precipitate or mud, the application of black to the hat-forms begins, I fear, to a.s.sume at length a too close a.n.a.logy to another blacking process closely a.s.sociated with a pair of brushes and the time-honoured name of Day & Martin. With that logwood black fibre, anyone could argue as to a considerable proportion of the dye rubbing, wearing, or washing off. Thus, then, we have the second cause of the deterioration of the black, for the colour could not go into the fibre, and so it was chiefly laid or plastered on. You can also see that a logwood black hat dyer may well make the boast, and with considerable appearance of truth, that for the purposes of the English hat manufacturers, logwood black dyeing is the most appropriate, _i.e._ for the dyeing of highly proofed and stiff goods, but as to the permanent character of the black colour on those stiff hats, there you have quite another question. I firmly believe that in order to get the best results either with logwood black or "aniline blacks," it is absolutely necessary to have in possession a more scientific and manageable process of proofing. Such a process is that invented by F.W. Cheetham (see Lecture VII. p. 66).

In the dyeing of wool and felt with coal-tar colours, it is in many cases sufficient to add the solution of the colouring matters to the cold or tepid water of the dye-bath, and, after introducing the woollen material, to raise the temperature of the bath. The bath is generally heated to the boiling-point, and kept there for some time. A large number of these coal-tar colours show a tendency of going so rapidly and greedily on to the fibre that it is necessary to find means to restrain them. This is done by adding a certain amount of Glauber"s salts (sulphate of soda), in the solution of which coal-tar colours are not so soluble as in water alone, and so go more slowly, deliberately, and thus evenly upon the fibre. It is usually also best to dye in a bath slightly acid with sulphuric acid, or to add some bisulphate of soda.

There is another point that needs good heed taking to, namely, in using different coal-tar colours to produce some mixed effect, or give some special shade, the colours to be so mixed must possess compatibility under like circ.u.mstances. For example, if you want a violet of a very blue shade, and you take Methyl Violet and dissolve it in water and then add Aniline Blue also in solution, you find that precipitation of the colour takes place in flocks. A colouring matter which requires, as some do, to be applied in an acid bath, ought not to be applied simultaneously with one that dyes best in a neutral bath. Numerous descriptions of methods of using coal-tar dyestuffs in hat-dyeing are available in different volumes of the _Journal of the Society of Chemical Industry_, and also tables for the detection of such dyestuffs on the fibre.

Now I will mention a process for dyeing felt a deep dead black with a coal-tar black dye which alone would not give a deep pure black, but one with a bluish-purple shade. To neutralise this purple effect, a small quant.i.ty of a yellow dyestuff and a trifle of indigotin are added. A deep black is thus produced, faster to light than logwood black it is stated, and one that goes on the fibre with the greatest ease. But I have referred to the use of small quant.i.ties of differently coloured dyes for the purpose of neutralising or destroying certain shades in the predominating colour. Now I am conscious that this matter is one that is wrapped in complete mystery, and far from the true ken of many of our dyers; but the rational treatment of such questions possesses such vast advantages, and pre-supposes a certain knowledge of the theory of colour, of application and advantage so equally important, that I am persuaded I should not close this course wisely without saying a few words on that subject, namely, the optical properties of colours.

Colour is merely an impression produced upon the retina, and therefore on the brain, by various surfaces or media when light falls upon them or pa.s.ses through them. Remove the light, and colour ceases to exist. The colour of a substance does not depend so much on the chemical character of that substance, but rather and more directly upon the physical condition of the surface or medium upon which the light falls or through which it pa.s.ses. I can ill.u.s.trate this easily. For example, there is a bright-red paint known as Crooke"s heat-indicating paint. If a piece of iron coated with this paint be heated to about 150 F., the paint at once turns chocolate brown, but it is the same chemical substance, for on cooling we get the colour back again, and this can be repeated any number of times. Thus we see that it is the peculiar physical structure of bodies which appear coloured that has a certain effect upon the light, and hence it must be from the light itself that colour really emanates. Originally all colour proceeds from the source of light, though it seems to come to the eye from the apparently coloured objects.

But without some elucidation this statement would appear as an enigma, since it might be urged that the light of the sun as well as that of artificial light is white, and not coloured. I hope, however, to show you that that light is white, because it is so much coloured, so variously and evenly coloured, though I admit the term "coloured" here is used in a special sense. White light contains and is made up of all the differently coloured rainbow rays, which are continually vibrating, and whose wave-lengths and number of vibrations distinguish them from each other. We will take some white light from an electric lantern and throw it on a screen. In a prism of gla.s.s we have a simple instrument for unravelling those rays, and instead of letting them all fall on the same spot and illumine it with a white light, it causes them to fall side by side; in fact they all fall apart, and the prism has actually a.n.a.lysed that light. We get now a coloured band, similar to that of the rainbow, and this band is called the spectrum (see Fig. 16). If we could now run all these coloured rays together again, we should simply reproduce white light. We can do this by catching the coloured band in another prism, when the light now emerging will be found to be white.

Every part of that spectrum consists of h.o.m.ogeneous light, _i.e._ light that cannot be further split up. The way in which the white light is so unravelled by the prism is this: As the light pa.s.ses through the prism its different component coloured rays are variously deflected from their normal course, so that on emerging we have each of these coloured rays travelling in its own direction, vibrating in its own plane. It is well to remember that the bending off, or deflection, or refraction, is towards the thick end of the prism always, and that those of the coloured rays in that a.n.a.lysed band, the spectrum, most bent away from the original line of direction of the white light striking the prism, are said to be the most refrangible rays, and consequently are situated in the most refrangible end or part of the spectrum, namely, that farthest from the original direction of the incident white light. These most refrangible rays are the violet, and we pa.s.s on to the least refrangible end, the red, through bluish-violet, blue, bluish-green, green, greenish-yellow, yellow, and orange. If you placed a prism say in the red part of the spectrum, and caught some of those red rays and allowed them to pa.s.s through your prism, and then either looked at the emerging light or let it fall on a white surface, you would find only red light would come through, only red rays. That light has been once a.n.a.lysed, and it cannot be further broken up. There is great diversity of shades, but only a limited number of primary impressions. Of these primary impressions there are only four--red, yellow, green, and blue, together with white and black. White is a collective effect, whilst black is the ant.i.thesis of white and the very negation of colour. The first four are called primary colours, for no human eye ever detected in them two different colours, while all of the other colours contain two or more primary colours. If we mix the following tints of the spectrum, _i.e._ the following rays of coloured light, we shall produce white light, red and greenish-yellow, orange and Prussian blue, yellow and indigo blue, greenish-yellow and violet. All those pairs of colours that unite to produce white are termed complementary colours. That is, one is complementary to the other. Thus if in white light you suppress any one coloured strip of rays, which, mingled uniformly with all the rest of the spectral rays, produces the white light, then that light no longer remains white, but is tinged with some particular tint. Whatever colour is thus suppressed, a particular other tint then pervades the residual light, and tinges it. That tint which thus makes its appearance is the one which, with the colour that was suppressed, gave white light, and the one is complementary to the other. Thus white can always be compounded of two tints, and these two tints are complementary colours.

But it is important to remark here that I am now speaking of rays of coloured light proceeding to and striking the eye; for a question like this might be asked: "You say that blue and yellow are complementary colours, and together they produce white, but if we mix a yellow and a blue paint or dye we have as the result a green colour. How is this?"

The cases are entirely different, as I shall proceed to show. In speaking of the first, the complementary colours, we speak of pure spectral colours, coloured rays of light; in the latter, of pigment or dye colours. As we shall see, in the first, we have an addition direct of coloured lights producing white; in the latter, the green colour, appearing as the result of the mixture of the blue and yellow pigments, is obtained by the subtraction of colours; it is due to the absorption, by the blue and yellow pigments, of all the spectrum, practically, except the green portion. In the case of coloured objects, we are then confronted with the fact that these objects appear coloured because of an absorption by the colouring matter of every part of the rays of light falling thereupon, except that of the colour of the object, which colour is thrown off or reflected. This will appear clearer as we proceed. Now let me point out a further fact and indicate another step which will show you the value of such knowledge as this if properly applied. I said that if we selected from the coloured light spectrum, separated from white light by a prism, say, the orange portion, and boring a hole in our screen, if we caught that orange light in another prism, it would emerge as orange light, and suffer no further a.n.a.lysis. It cannot be resolved into red and yellow, as some might have supposed, it is monochromatic light, _i.e._ light purely of one colour. But when a mixture of red and yellow light, which means, of course, a mixture of rays of greater and less refrangibility respectively than our spectral orange, the monochromatic orange--is allowed to strike the eye, then we have again the impression of orange. How are we to distinguish a pure and monochromatic orange colour from a colour produced by a mixture of red and yellow? In short, how are we to distinguish whether colours are h.o.m.ogeneous or mixed? The answer is, that this can only be done by the prism, apart from chemical a.n.a.lysis or testing of the substances.

[Ill.u.s.tration: FIG. 16.]

The spectroscope is a convenient prism-arrangement, such that the a.n.a.lytical effect produced by that prism is looked at through a telescope, and the light that falls on the prism is carefully preserved from other light by pa.s.sing it along a tube after only admitting a small quant.i.ty through a regulated slit.

Now all solid and liquid bodies when raised to a white heat give a continuous spectrum, one like the prismatic band already described, and one not interrupted by any dark lines or bands. The rays emitted from the white-hot substance of the sun have to pa.s.s, before reaching our earth, through the sun"s atmosphere, and since the light emitted from any incandescent body is absorbed on pa.s.sing through the vapour of that substance, and since the sun is surrounded by such an atmosphere of the vapours of various metals and substances, hence we have, on examining the sun"s spectrum, instead of coloured bands or lines only, many dark ones amongst them, which are called Fraunhofer"s lines. Ordinary incandescent vapours from highly heated substances give discontinuous spectra, _i.e._ spectra in which the rays of coloured light are quite limited, and they appear in the spectroscope only as lines of the breadth of the slit. These are called line-spectra, and every chemical element possesses in the incandescent gaseous state its own characteristic lines of certain colour and certain refrangibility, by means of which that element can be recognised. To observe this you place a Bunsen burner opposite the slit of the spectroscope, and introduce into its colourless flame on the end of a platinum wire a little of a volatile salt of the metal or element to be examined. The flame of the lamp itself is often coloured with a distinctiveness that is sufficient for a judgment to be made with the aid of the naked eye alone, as to the metal or element present. Thus soda and its salts give a yellow flame, which is absolutely yellow or monochromatic, and if you look through your prism or spectroscope at it, you do not see a coloured rainbow band or spectrum, as with daylight or gaslight, but only one yellow double line, just where the yellow would have been if the whole spectrum had been represented. I think it is now plain that for the sake of observations and exact discrimination, it is necessary to map out our spectrum, and accordingly, in one of the tubes, the third, the spectroscope is provided with a graduated scale, so adjusted that when we look at the spectrum we also see the graduations of the scale, and so our spectrum is mapped; the lines marked out and named with the large and small letters of the alphabet, are certain of the prominent Fraunhofer"s lines (see A, B, C, a, d, etc., Fig. 16). We speak, for example, of the soda yellow-line as coinciding with D of the spectrum.

These, then, are spectra produced by luminous bodies.

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 pa.s.s 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 gla.s.s full of picric acid in water, and of a yellow colour. Now when I pa.s.s 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 pa.s.s 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 pa.s.s 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 pa.s.sing 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 pa.s.sing 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 pa.s.s 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 pa.s.s 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 a.n.a.logy 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 pa.s.s 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 versa_; 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.

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