See RESEDA, BOTANY AND DYEING INDEX.

DYEING.

Definition. 1. Dyeing is the art of communicating a permanent colour to any substance; but it is generally employed in a more limited sense, and is applied to the art of giving colours to wool, silk, cotton or flax, or to thread or cloth fabricated of these substances. To this more limited sense we propose to confine it in the following treatise; and for the dyeing or staining of other substances, as paper, wood, bone, leather, marble, the reader is referred to these articles.

Origin of arts:

2. Among the arts of life there are some which are essential to man even in the earliest period of his history; while others derive their origin from chance, and owe their improvement and perfection to the progress of refinement and luxury. Those arts which are connected with the means of providing food or shelter are necessary even in the rudest state of man, and are practised with more or less dexterity and success according to the abundance or scantiness of the supply with which he is furnished, and the varieties of climate which he inhabits. But those arts which have been distinguished by the name of fine arts can only flourish and arrive at a high degree of perfection in the more luxurious ages of refined society. To this account of the origin and progress of the arts among mankind the art of dyeing forms a remarkable exception. Totally unconnected with the means of providing food to satisfy the urgent calls of hunger, of preparing raiment to secure the body from cold, or of procuring shelter from the storm, this art might at first sight be considered as one of those which exclusively belong belong to an age of luxury. But the history of mankind affirms its origin a very different period. The art of dyeing seems to be almost co-eval with man. In the rudest state of his existence, his simple and scanty clothing is frequently coloured; and even the naked savage, while he is yet a housetlefs wanderer in the woods, has discovered the means of staining his body with different colours. And yet the art of dyeing in no respect contributes to relieve the real and primary wants of man. It renders not his raiment warmer, and it serves not to make his lodging more comfortable.

3. Whence then is the origin of this art? It depends not like others on the necessities of man, and it exists long before he is acquainted with refinement and luxury. It must therefore be traced to a different source.

We see that the desire of distinction is one of the most active principles in the human mind. This principle operates equally in the breast of the savage in the midst of his naked companions, and in that of the sage and the soldier in polished society. But man rarely rests satisfied with the solid, but frequently less obvious pre-eminence, which superior strength, genius, or learning confers. The proofs of this superiority, can be but seldom exhibited; they are often not generally understood or acknowledged, and therefore cannot always be fairly estimated. He who possesses any of those talents which give him a superiority to others, naturally wishes to be distinguished by certain marks by which he may more uniformly and more directly excite admiration and command respect. He seeks, therefore, for some adventitious circumstances which may be regarded as a kind of symbolical representation of power and greatness; and as they are constantly present to the senses, they make a deeper impression, and keep alive those feelings of admiration which are so gratifying to the vain and ambitious. Dyes and its ornaments have been usually employed as external marks of distinction. Hence it is, that the chief or the warrior among rude nations is clothed with a finer and more beautiful skin; his head is decorated with flowers or feathers; or the leaves of the oak, or the laurel, simply adorn his brow. And in the progress of civilization and refinement, the diadem of gold, and the robe of purple or of scarlet, supplant these simpler decorations as characteristics of dignity and power. To increase still more the beauty and variety of those substances which are employed as clothing or as the ornaments of dyes, the aid of colours has been called in; and accordingly we find that coloured clothing has been held in high estimation in all ages. This principle, therefore, the desire of distinction, seems to be the natural origin of the art of dyeing. Nature, however, furnishes the model, and may be regarded as the antetype of the art, in the gay plumage with which she has clothed the feathered tribes, and in the splendid colours and infinite variety of shades which are exhibited in her vegetable productions.

4. But without indulging farther in these speculations, which are to be considered as subjects of curious investigation, rather than as topics of practical utility, let us now take a short view of the history and progress of this art.

We have endeavoured to show that the beauty of brilliant colours is one of the means of attracting attention, and of acquiring distinction, which mankind in every period of society have employed. Even before the use of clothing has been introduced, the rude inhabitants of savage nations applied them first to their skins. This practice existed among the Britons in the time of Caesar; and the women of Gaul about the same period stained themselves of a brown olive colour. At this day, it is still the practice of many of the savage tribes of America, as well as of the natives of the South sea islands. But when mankind had made some progress in arts and civilization, and had begun to wear clothing, the colours which they admired were afterwards communicated to their garments. The art of dyeing, therefore, though in a rude and imperfect state, is indisputably of great antiquity; and indeed, considering its nature and origin, this might have been expected.

5. India, the nursery of the arts and sciences, which in India were afterwards improved and brought to perfection among other nations, seems to have given birth to the art of dyeing; and it would appear that the knowledge of dyeing cotton had advanced as far in the time of Alexander the Great as at the present time, so stationary have the arts become in that country. The beautiful colours of the Indian linens would naturally lead to the supposition that the art had reached a very high degree of perfection; but it is known that the Indian processes are so tedious, complicated and imperfect, that they would be totally impracticable in any other country.

6. It was not till the time of Alexander the Great among the Greeks, that the art of dyeing cotton and linen, which had gradually spread from the east to the west, was known in Europe. The Greeks, however, as appears from many passages in the Iliad and Odyssey, were acquainted with the art of dyeing purple in the time of Homer. And it is supposed that they derived their knowledge of it from the Phoenicians, a people who were very early celebrated for the art of dyeing. But their art seems to have been confined to wool; silk, indeed, was at that time unknown, and linen was usually worn white.

7. Dyeing and coloured stuffs are frequently mentioned in the sacred writings. It would appear that the art had made considerable progress in the time of the patriarchs, from what is mentioned in the book of Genesis. The dyed stuffs which are described in the book of Exodus were purchased by the Jews from the Phoenicians.

8. The Egyptians according to Pliny, practised a kind of topical dyeing or calico-printing, which from his general description seems to have been similar to that which was found many ages after to exist in different parts of India, and was from thence introduced into the different countries of Europe. He says, the Egyptians began by painting on white cloths, which were no doubt of linen or cotton, with certain drugs which were themselves colourless, but possessed the property of absorbing colouring substances. These cloths were afterwards immersed in a heated dyeing liquor which was of one uniform colour, and although they were formerly colourless, yet when they were taken out, they were found to be dyed of different colours, according to the different qualities of the substances which had been applied to their different parts; and these colours could not afterwards be discharged by This art was probably borrowed from the natives of India.

The Tyrian purple, so celebrated among the ancients, was probably from the name discovered at Tyre, and perhaps contributed not a little to the opulence of that city. The liquor which was employed in dyeing the purple was extracted from two kinds of shell-fish, one of which, the larger, was called the purple, and the other was a species of whelk. Each of these species was subdivided into different varieties, and were otherwise distinguished, according to the places where they were found, and as they yielded more or less of a beautiful colour. It is in a vessel in the throat of the fish that the colouring liquor is found. Each preparation fish only afforded a single drop. When a certain quantity of the liquor had been obtained, it was mixed with a proportion of common salt, macerated together for three days, and five times the quantity of water was added. The mixture being kept in a moderate heat, the animal parts which happened to be mixed with it, separated and rose to the surface. At the end of ten days, when these operations were finished, a piece of white wool was immersed, by which means they ascertained whether the liquor had acquired the proper shade.

Various processes were followed to prepare the stuff to receive the dye. By some it was immersed in lime water, and by others it was prepared with a kind of ficus, which acted as a mordant to give it a more fixed colour. Alkanet was used by some for the same purpose.

The liquor of the whelk did not alone yield a durable colour. The liquor from the other shell-fish served to increase its brightness; and thus two operations were in use to communicate this colour. A first dye was given by the liquor of the purple, and a second by that of the whelk; from which it was called by Pliny purpura dibapha, or purple twice dipped.

Some kinds of purple have been found to possess great durability. Plutarch, in his life of Alexander the Great, mentions that the Greeks discovered in the treasury of the king of Persia a great quantity of purple which was 190 years old, and still retained all its beauty.

The small quantity of liquor which could be obtained from each shell-fish, and the tedious process in its preparation and application to the stuffs, raised the price of purple so high, that in the time of Augustus a pound of wool of the Tyrian purple dye, could not be purchased for one thousand denarii, equal to about 36l. sterling.

The purple, which has been almost everywhere a mark of distinction attached to high birth and dignity, was worn by those who held the first offices in Rome. The emperors at last referred to themselves the right of wearing it, and prohibited all others from using it on pain of death.

The substances which have been discovered and used in dyeing by the moderns, and the superiority which they have obtained in many colours, have superseded the use of the purple of the ancients. The shell-fish from which the liquor is extracted, is supposed to be now as abundant as ever. Similar shell-fish have been found near Nicoya, a small Spanish town in South America, and they are at present used for dyeing cotton on the coasts of Guayaquil and Guatemala.

In the year 1683, Mr Cole of Bristol discovered, on the coast of England, the shell-fish which yields the purple liquor. The liquor was contained in a white vein, lying transversely in a little furrow or cleft, next to the head of the fish. He found by experiment, that letters or marks, made with this white liquor, appeared when first exposed to the air of a green colour. When exposed to the sun, it became of a deeper green, afterwards of a purplish red, and, by the continued action of the sun's rays, of a deep purple red. Mr Cole sent some of the first linen marked with this liquor to Dr Plot, one of the secretaries of the Royal Society, in the year 1684. It was soon after shown to King Charles II, who greatly admired it, and desired that some of the shell-fish might be collected and brought to town, that he might have an opportunity of seeing the liquor applied, and the successive changes of colour through which it passed.

A species of this shell-fish was also found by Phumer at the Antilles; and Reaumur made a number of experiments on whelks, which were collected on the coast of Poitou. Duhamel found the same shell-fish in great abundance on the coast of Provence. The experiments of these philosophers on this liquor afforded the same result as those of Mr Cole. They observed that, although at first white, it becomes by the action of light, of a yellowish green, then deepens to a kind of blue, which is afterwards changed to a red. In less than five minutes, the latter is converted into a fine deep purple, having all the characters of the purple of the ancients.

Eudocia Macrembolitissa, daughter of the emperor Constantine VIII, who lived in the 11th century, while the knowledge and practice of dyeing that colour for the use and at the expense of the Greek emperors still subsisted, has given a minute account of the mode of catching the shell-fish which produced the purple. Of this operation she herself, it would appear, was an eyewitness. As it was applied at that time, it did not acquire its full lustre and perfection of colour, till it had been exposed to the action of the sun's rays.

A liquor which yields the same colour, and has purple-like otherwise similar properties, is found in different parts of the world. Abundance of purple snails, it is said, are found in the islands opposite to Batavia. They are boiled and eaten by the Chinese, who polish the shells, and pick out of the middle of the snail a purple-coloured substance, which they use in colouring and making red ink. Dr Peyronel describes what he calls the naked snail, which is found in the seas of the Antilles, and affords a liquor of a beautiful purple colour. This liquor is thrown out by the animal when it is disturbed, in the same way as the cuttle-fish discharges the ink. The liquor of the snail is naturally of a purple colour, without the application of light.

Two shell-fishes, which yield a similar colouring liquor, are described by Dr Brown in his history of Jamaica. The one, he says, is frequent in the American seas, and emits on being touched a considerable quantity of viscid purple liquor, which thickens and colours water. The other is called the purple ocean shell, and yields a beautiful purple liquor, which seems to resemble the former. But investigations concerning ing the nature and application of the purple dye from shell-fish are now to be considered merely as subjects of curiosity; because the colours which are obtained by the processes of the moderns are more beautiful, and far less expensive.

16. In the 5th century, during the irruption of the northern barbarians, the arts, which had been encouraged and protected by the Romans, were lost amidst the devastations of the western empire. A few, indeed, were preserved in Italy, but they were in a state of decay; and otherwise no traces remained of knowledge, industry, or humanity. A manuscript of the 8th century is quoted by Muratori, which contains a description of some dyes, principally for skins, as well as some processes connected with other arts; but from the barbarous Latin, in which it is written, no distinct notion can be formed of the nature of these processes. The arts met with a better fate in the East, where they were protected and encouraged. So late as the 12th century, articles of luxury were procured by some of the great from eastern countries.

17. During the time of the crusades, Venice and other cities of Italy became rich and powerful, first by supplying with provisions the Europeans who engaged in these frantic and destructive expeditions, and afterwards by establishing an intercourse with the Grecian empire. By these means the arts, which had been preserved among the Greeks, were established in Italy. In the year 1338, the city of Florence contained 200 manufacturers, who are said to have produced from 70,000 to 80,000 pieces of cloth. In the year 1300, archil was accidentally discovered by a Florentine merchant. Observing that urine produced a fine colour on certain species of moss, he made experiments, and from these learned the mode of preparing this substance. The discovery was long kept secret. His posterity, a branch of which, it is said, still exists, have retained the appellation of Rucellai, from the Spanish word, which signifies that kind of moss.

18. The arts, after being revived in Italy, continued for a long time to be cultivated and improved with increasing success. Along with these, the art of dyeing made considerable progress. The first collection of the processes employed in this art appeared at Venice in the year 1429. It was entitled Mariagola del' arte de i tintori. To render this description more useful and extensive, a person of the name of Giovanni Ventura Roffetti, travelled through different parts of Italy and the neighbouring countries, where the arts had begun to flourish, that he might acquire a knowledge of the processes which were employed by different dyers. These were collected and published in 1548, under the title of Picchio. This treatise has been by some considered as the leading step towards the perfection which the art of dyeing has attained; for it is the first in which the different processes are collected. No mention is made, either of cochineal or of indigo, so that it would appear, these dyes were either not known, or not employed in Italy previous to the time in which it was written.

19. Italy, but especially Venice, for a long time almost exclusively possessed the art of dyeing, and this seems to have contributed greatly to the prosperity of the manufactures and commerce which the Italian states long enjoyed. By degrees it was introduced into France, Holland, and Britain. The process for dyeing the true scarlet had been communicated to a person of the name of Gobelin, who established a manufacture in the north near Paris, which still bears his name. At the same time, this was considered so rare an enterprise, that it &c., received the name of Gobelin's folly; but such was his success, and such the ignorance of the times, that it was supposed he derived his knowledge of the processes he employed, from the devil!

20. The discovery of America brought the knowledge of the cochineal insect into Europe. The Spaniards observing that the Mexicans employed it in painting their houses, and in dyeing cotton, transmitted an account of the beauty of that colour to their government, whose attention was afterwards directed to encourage and promote the increase of the valuable insect from which it is obtained. The discovery of cochineal was soon followed by that of the process for dyeing scarlet, by means of a solution of tin. For this discovery we are indebted to a German chemist of the name of Kutter, or Kusler, who carried the secret to London in the year 1643. Gluck or Kloek, a Flemish painter, having obtained possession of this secret, communicated it to Gobelin, and afterwards the knowledge of it spread throughout all Europe. The use of indigo, which was a great acquisition to the art of dyeing, was more slowly established than that of cochineal. In the reign of Queen Elizabeth, the use of this substance, as well as of logwood, was strictly prohibited in England, and if found in any factory, was ordered to be burned. This, as must appear at the present time, very strange prohibition, was not withdrawn till the reign of Charles II. It met with the same fate in Saxony. In the edict in which the use of it is forbidden, it is said to be a corrosive colour, and called food for the devil!!

21. In France also, some prejudice was entertained against it, and although it was not entirely prohibited, the use of it was limited to a certain proportion. The reasons on which this prejudice was founded, on a narrow view of the principles of political economy, might even in the present day be admitted as specious, if not satisfactory. It was held out by those who dyed blue, and were accustomed to use pastil and woad, that the introduction of indigo would supersede the use of these substances; and it was represented that their consumption would be destroyed, and the encouragement for the productions of the country diminished.

22. Previous to the administration of the celebrated Colbert, the industry and arts of France long remained in a state of languor and decay. By the wise measures which were adopted by this minister, the soon rose to distinction among the nations of Europe, and in a short time saw her commerce and manufactures greatly extended. He invited the most skillful artists, encouraged and rewarded their talents, and thus established many arts and manufactures. Among these, the art of dyeing received its share of attention. In the year 1672, he published a table of instructions for dyeing, which, although it contains many useless and improper restrictions, is on many accounts worthy of attention, and particularly the reasons which he has given for considering it as an object of consequence. As a proof of this, we may refer to the following extract: tract from the instructions: "If, it is said, the manu- factories of silk, wool, and thread, are to be reckoned among those which most contribute to the support of commerce; dyeing, which gives them that striking va- riety of colour, by which they resemble what is most beautiful in nature, may be considered as the soul of them, without which a body could scarcely exist.

"Wool and silk, the natural colour of which rather indicates the rudeness of former ages, than the genius and improvement of the present, would be in no great request, if the art of dyeing did not furnish attractions which recommend them, even to the most barbarous nations. All visible objects are distinguished and recommended by colours; but for the purposes of commerce, it is not only necessary that they should be beautiful, but that they should be good, and that their duration should equal that of the materials which they adorn."

23. But notwithstanding these just and liberal views, and many useful regulations, which were published for instructions in the art of dyeing, the restrictions imposed upon it, as we have already observed, were, from mistaken views improper and injurious, because in this, as in every other art, these restraints infallibly operate as checks on industry and improvement. The effects of these prohibitions, however, were moderated by the facility with which they might be eluded, and by the rewards bestowed on those whose experiments pro- moted the progress of the art, and whose discoveries being afterwards published, served to modify the exist- ing regulations. The effects of these prohibitions, too, were in a great measure obviated, by the judi- cious appointment of men of science, to whom the superintendence of arts and manufactures was en- trusted. By their prudent exertions, and by the still more efficacious means of the diffusion of knowledge, this art, as well as others, has been encouraged and improved.

24. The French government continued to direct its at- tention to promote the plan which was thus begun by Colbert, and many eminent chemists have been em- ployed to superintend and improve the processes of the art of dyeing. Dufay, Hellot, Macquer, and Berthol- let, have been successively charged with the care of this department; and to their labours and exertions we are indebted for many valuable acquisitions which have been made in the art of dyeing, during the 18th century. Dufay was the first who entertained just views of the nature of colouring matters, and the powers by which they adhere. In the examination of certain pro- cesses he discovered great sagacity, and established the surest means which the state of knowledge at the time afforded, to ascertain the durability of a colour. Under his direction a new table of instructions, which superseded that of Colbert, was published in 1737. Hellot, who succeeded him, published in 1740 a me- thodical description of the processes for dyeing wool; and this treatise may be considered, even at the pre- sent day, as one of the best systems on the subject. Macquer in 1763 published a treatise on dyeing silk, in which he has given an accurate description of the processes, has discovered the combinations of the col- ouring principle of Prussian blue, and has endeavoured to make an application of it to the art of dyeing. Macquer died in 1784, and was succeeded in that de- partment by the celebrated Berthollet, to whom was intrusted the superintendence of the arts connected with chemistry, and particularly that of dyeing. To his being placed in this department, we are probably indebted for the excellent work which he has publish- ed on this subject, and for different memoirs which have appeared in different periodical works. To these we must acknowledge ourselves greatly indebted for much of the information both of the theory and prac- tice of this art, which we propose to lay before our readers in the following treatise. He has endeavoured himself, to bring into one point of view the proce- dices of industry, and the operations of nature; to take his situation between the philosopher and the ar- tist. To the first he has shown, where it is that the phenomena of the art of dyeing and those of nature meet, and what are the principles which their disco- veries have established. When these comprehensive views, we may add, are completed, the art of dyeing may be considered as perfect.

25. The art of dyeing has been long successfully practised in Britain, although little has been done to- wards the investigation of the theory on which it depends. At an early period of the Royal Society, it attracted the attention of some of its members; but nothing was published on the subject. Many years afterwards, some useful observations on dyeing were published by Dr Lewis, but these were limited to a very few processes. The only work with which the British dyers were acquainted, till within these few years, was a translation of the treatise of Hellot, men- tioned above.

26. But since the progress of chemical science has improved opened so wide a field of investigation; and since all by chemi- the essential processes in the art of dyeing are to be considered as purely chemical, the attention of philolo- phers has been greatly occupied with its investigation and improvement. By their experiments and observa- tions a great deal of new information has been accu- mulated, and much new light has been thrown upon the art.

27. The only treatise which has appeared in Sweden on this subject, is that of Scheffer, accompanied with notes by the celebrated Bergman. In Germany, experiments in different processes of dyeing have been published by Beckmann, Poerner, Vogler, and Francheville. The authors of the different treatises in France on this sub- ject, which have greatly contributed to the improve- ment of the art, are D'Ambourney, D'Apligny, Hauffmann, Chaptal, and Berthollet, whose works we have already mentioned. In Britain, two very valu- able essays by Delaval and Henry have appeared; and to these we may add, the excellent treatise on the Phi- losophy of Permanent Colours, by Dr Bancroft.

In the following treatise, we propose to give a pretty full view, both of the theory and practice of dyeing. This subject naturally divides itself into two parts. In the first, we shall treat of dyeing in general, or of those departments of physical science, the knowledge and application of which may be con- sidered as constituting the theory of the art. In the second part, we shall take a view of the different pro- cesses which are employed in communicating colours to different stuffs, or, in general terms, the practice of dyeing. PART I. OF DYEING IN GENERAL.

UNDER this head we propose to take a general view of what may be regarded as the theory of dyeing; and investigate those principles of physical science which are immediately connected with the art, and by the application of which the phenomena of the art can only be accounted for, or satisfactorily explained. With this view we shall treat the subjects which come under this part in the four following chapters. In the first, we shall consider the nature of colours and colouring matters; in the second, we shall treat of the nature and operation of mordants; the third will include an account of the properties of the substances to which colours are communicated; and, in the fourth, we shall add some general observations on the operations of dyeing.

CHAP. I. Of Colours and Colouring Matters.

28. The physical theory of light and vision properly belongs to optics, and the changes produced by the action of light on different substances, are detailed under chemistry. In this place, therefore, we shall only make a few observations on the nature of light and colours, which are more immediately connected with the subject under consideration. For our knowledge of light and vision we are indebted to Sir Isaac Newton. It was first demonstrated by that sagacious philosopher, that the light of the sun is composed of seven rays which have different powers of refrangibility. The colours of these seven rays are red, orange, yellow, green, blue, indigo, violet. When these rays are separated by the prism, as they run gradually into each other, according to their different degrees of refrangibility, they produce every various shade of colour. The violet ray is the most refracted, the indigo, next, and so on to the red, which is the least refracted of all the rays. The same rays of light also differ in their degrees of reflexivity. All known colours, and their different shades, are produced by mixing together the different rays. Thus, for instance, by mixing together red and yellow, an orange colour is obtained; yellow and blue give a green colour; and blue and red, according to their different proportions, produce a violet, purple, &c., and thus, as Sir Isaac Newton has observed, the variety of colours depends on the composition of light; for if the sun's light consisted but of one sort of rays, there would be but one colour.

29. Colours in an object, the same philosopher farther observes, are nothing but a disposition to reflect this or that sort of rays more copiously than the rest; in the rays they are nothing but their dispositions to propagate this or that motion into the fenatorium; and in the fenatorium they are sensations of those motions under the forms of colours. In their power of reflecting light, bodies, it is well known, differ greatly from each other. Some bodies reflect it in such quantities, that the eye cannot bear it. Such, for instance, are metallic substances highly polished. Others again, as dark-coloured or black substances, reflect it very feebly. It is found in general, that the quantity of light reflected from a body depends greatly on the smoothness of its surface. On this account bodies which have the smoothest surface, or are most highly polished, are the brightest; that is, they reflect the greatest quantity of light. But there is also a very great difference among bodies, in the nature or quality of the rays of light which they have the power of reflecting. When all the rays of light are equally reflected by any body, that body is said to be white; but when a very few rays only are reflected from a body, that body is said to be black, because the greater number of the rays being absorbed by the body, the few that are reflected make a very faint impression on the organ of vision. A body which has the power of reflecting the red rays only, is said to be red; a body which reflects the blue rays, is said to be blue; the body reflecting only the yellow rays, is yellow; but when any two of these rays are reflected in combination with each other, a different colour is produced; as for instance, the red and the yellow rays afford an orange colour; and as we have already observed, the various shades of colour exhibited by different bodies, depend on the different combinations of rays reflected from their surface. Thus it appears, that colour in bodies is to be ascribed to their disposition of absorbing certain rays, and reflecting the rest. In opaque bodies, it is owing to their disposition to absorb some rays, and to reflect the rest. In transparent bodies, it is owing to their disposition to absorb certain rays, and to transmit the rest.

30. Newton has demonstrated, that transparent Cause of bodies reflect the rays of one colour, and transmit those colours instead of another, according to the difference of their thicknesses or density. He observes that transparent substances, such as glass, water, air, &c., when made very thin by being blown into bubbles, or otherwise formed into plates, exhibit various colours, according to their various thicknesses; although at a greater thickness they appear very clear and colourless. His method of conducting these experiments was the following. He took two object-glasses, the one a plano-convex for a 14 feet telescope, and the other a large, double convex, for one of about 50 feet; and upon this laying the other with its plain side downwards, he pressed them slowly together, to make the colours successively emerge in the middle of the circles, and then slowly lifted the upper glass from the lower, to make them successively vanish again, in the same place. The colour which, by pressing the glasses together, emerged last in the middle of the other colours, would, upon its first appearance, look like a circle of a colour almost uniform from the circumference to the centre; and by compressing the glasses still more, grow continually broader, until a new colour emerged in its centre, and thereby it became a ring, encompassing that new colour; and by compressing the glasses still more, the diameter of this ring would increase, until another new colour emerged in the centre of the last, and so on, until a third, a fourth, a fifth, and other following new colours successively emerged there, and became Of Colours, came rings, encompassing the innermost colour, &c., last of which was the black spot. And on the contrary, by lifting up the upper glass from the lower, the diameter of the rings would decrease, and the breadth of their orbit increase, until their colours reached successively to the centre, and then, as they were of considerable breadth, he could more easily discern their species than before. By proceeding in this manner, he produced 25 different-coloured, circular rings, which he divided into seven orders, because the same colour was always repeated. They are reckoned from the central colour, which was always black, in the following order:

1. Blue, white, yellow, and red. 2. Violet, blue, green, yellow, red. 3. Purple, blue, green, yellow, red. 4. Green, red. 5. Greenish blue, and red. 6. Greenish blue, and pale red. 7. Greenish blue and reddish white.

But in the three last orders the colours were very indistinct, and terminated in perfect whiteness.

31. These colours were occasioned by the thin films of air which were included between the two glasses. For he found, he observes, by looking through the two object-glasses, that the interjacent air exhibited rings of different colours, as well by transmitting light, as by reflecting it. The film of air varies in thicknesses from the centre of the glasses to the circumference. In the centre where the film is thinnest the colour is black; and the other colours from the centre to the circumference are produced in their order by the gradual increase of the thickness of the film.

32. These experiments were repeated on films of water and also of glass; and it was found that the thickness of the films in these cases, reflecting any particular colour, was diminished, and this diminution appeared to be proportional to the density of the reflecting film. As there is no method of measuring the distance between the two glasses where the black spot appears, it is impossible to ascertain the absolute thickness of the films; but it certainly does not exceed the thousandth part of an inch. Newton, however, endeavoured by a mathematical investigation to measure the relative thickness of air, water, and glass, at which the several orders of colour appear. The following table exhibits the relative thickness of air which produced the coloured circles.

| Order | Thickness (inches) | |-------|-------------------| | 1. Black | 1 | | | blue | 2½ | | | white | 5½ | | | yellow | 7½ | | | red | 8½ | | 2. Violet | 11½ | | | blue | 14 | | | green | 15½ | | | yellow | 16½ | | | red | 18½ | | 3. Purple | 21 | | | blue | 23½ |

33. The conclusion which Newton drew from these experiments was, that the power or disposition of the particles of bodies to reflect or transmit particular rays, Or Colours, &c., depended on the size and density of these particles; and proceeding on this theory he attempted to measure the size, or at least the thickness, of the particles of bodies, from the colours which they reflected or transmitted.

34. This subject was still farther investigated by Mr Supported Delaval. In the year 1765, he published, in the Philosophical Transactions, an account of his "Experiments and Observations on the agreement between the specific gravities of the several metals, and their colours, when united to glasses, as well as of their other preparations." In this paper, Mr Delaval treats of the difference of density, and of the colours produced by that cause; and yet he considers colours as arising from a difference of the size of the colouring particles. For since the particles of a coloured substance being separated they are removed to a greater distance from each other, and thus occupy a greater space, that substance must undergo a diminution of its specific gravity, while at the same time the size of its particles is smaller. According to Sir Isaac Newton, the refractive and reflective powers of bodies are nearly proportional to their densities, and the least refrangible rays require the greatest power to reflect them. From this, Mr Delaval supposed, that denser substances, by their greater reflective power, ought in similar circumstances to reflect the less refrangible rays; and that substances of less density should reflect rays proportionably more refrangible, and therefore appear of several colours in the order of their density. The densest bodies, he supposes, are the red; the next in density are the orange; the next are the yellow; and so on, according to the order of the refrangibility of the different rays. Mr Delaval some time after extended his researches to animal and vegetable substances, and endeavoured to establish the theory of Newton by a great number of experiments, an account of which he published in an essay entitled, An Experimental Inquiry into the cause of the Permanent Colours of Opaque Bodies.

35. According to the theory of Newton, with the exception of combustible bodies which follow a different law; colour depends solely upon the size of the integrant particles of bodies, in which the density is the same; and upon the size and density of all bodies taken together. But the evidence for the truth of this theory can only be derived from experiment. Newton deduced but a small number of experiments in support of it. The experiments of Mr Delaval were more numerous and more varied; but they were made long before the important facts in chemical science, which have completely changed the views and opinions of philosophers, with regard to the nature and action of the constituent principles of bodies, were discovered; so that it is now universally acknowledged that they proceeded on a false hypothesis. It was supposed that alkalies enlarge, and that acids diminish, the size of the particles of bodies on which they act, without inducing any other change. This opinion, in the present state of chemical knowledge, will not readily find a place.

36. But if this theory were true, every change in the size of the integrant particles of bodies would occasion a different colour in these particles; and in all these changes, if they correspond with the theory, Of Colours, the colour produced must be precisely that colour which is the result of a diminution or increase of size.

Inconsistent with the facts.

37. But there is no such coincidence with the facts. The magnitude of the integrant particles of bodies cannot be ascertained; and there is no method by which the increase or diminution of the particles in the changes which they undergo can be measured; but the addition or abstraction of matter to particles can in many cases be distinctly determined. In the change which takes place on gold by the process of oxidation, that is, by combining with oxygen, an integrant particle of the oxide is larger than an integrant particle of gold in the metallic state; for it has united with one particle at least of oxygen. But if the theory were true, there should be a difference of colour between the oxide and the gold, which is not the case; for they are both yellow. In the amalgam of silver, a compound of silver and mercury, the colour is white, which is the colour of both metals; and yet an integrant particle of the compound must be larger than an integrant particle of either the mercury or the silver.

38. But the same colour, it may be said, is reflected in the different orders of colours, in which the particles are of very different sizes. This circumstance, as Dr Bancroft justly observes, proves incontrovertibly, that although thicknesses or sizes of the particles may be one, it cannot be the only cause of the repeated variation of colour. It follows, therefore, that there must be some other cause. But besides, the most common colour remaining after an increase of the size of the integrant particles of bodies is white; and yet this colour does not appear in any of the orders except the first; its permanency, therefore, cannot be accounted for in any way which is at all compatible with this theory.

39. And in the changes of colour which are observed to follow the increase or diminution of the sizes of the particles of bodies, the order of these changes is not such as will correspond with the theory. It is obvious that the colours of metallic substances do not depend on their density. The colour of platinum, the densest body known, is not red, as it should be, according to the theory, but white; in this respect resembling tin, one of the metals which has the least density, and little more than one third that of the former.

40. The size of the particles of the green oxide of iron must be increased when they enter into combination with the prussic acid. But the colour of the compound is white; and, according to the theory, it should be accompanied with a diminution of the size of the particles, which is not the case. The colour of indigo is naturally green. The addition of oxygen, which must increase the size of the particles, converts it to a blue colour. This, then, is another case incompatible with the Newtonian theory. And from these facts it must appear, that this theory is deficient in accounting for the reflection or transmission of particular rays, and the absorption of the rest. It is not sufficient for the explanation of the causes of colour. The smallness and the density of particles are not the only circumstances which ought to be taken into the account, in explaining the cause of colour in bodies. It appears, from Newton's own experiments, that we must have recourse to the chemical properties of bodies, which have a considerable influence on their colour. It cannot be supposed, that a force which acts powerfully in refracting the rays, will not also have great influence in their reflection.

41. Numerous facts tend to prove that bodies have affinity of a particular affinity for the rays of light; and indeed bodies for it is entirely upon these affinities that the phenomena certain of light depend. Coloured bodies have a certain affinity for some of the rays of light. Those rays for colour, which any body has a strong affinity, are absorbed by it, and retained; while the other rays, for which it has no affinity, are either reflected or transmitted, according to the nature of the body, as it may be opaque or transparent, and according to the direction of the incident ray. A red body, for instance, reflects only the red rays, because it has an affinity for all the other rays, excepting the red. It therefore absorbs them, if it be an opaque body, or transmits them if it be transparent. A green body absorbs all the rays excepting the green; a black body has a strong affinity for all the rays, and therefore they are all absorbed; while a white body, which has little affinity for any of the rays, if it be opaque, reflects, or if transparent, transmits them all.

42. The differences which exist between the particles of bodies, may be conceived to be differences in colour, size, density, and figure; and changes in these circumstances may account for all the varieties of affinity, in size, &c.; if then affinity depends upon these circumstances, and of the particles of bodies is to be ascribed to the affinities between their particles and the different rays of light, the cause of the colour of bodies, it seems obvious, is capable of being accounted for from the size, density, and figure of their particles. It cannot be accounted for, according to the theories of Newton and Delaval, solely on the variations in size and density.

43. If then the colour of bodies depends upon their affinity for light, and every body have some colour in consequence of the absorption of particular rays which it retains, and the reflection or transmission of all the rest, it is obvious, that it must continue of its first colour without suffering any change, till it is either saturated with the particular rays which it absorbs, or till the particles of the body have undergone some and to new change by decomposition or combination with new substances. As many substances have been long exposed to the action of light without their colours being changed, there is no certain evidence that the changes in the colours of bodies are to be ascribed to the first cause. The light which is absorbed either escapes unchanged or under some unknown form. But the action of the second cause which has been mentioned, may be traced in almost all cases where alterations of the colours of bodies have been observed. We may take as an example of this change of colour the green oxide of iron, which readily combines with oxygen, and is converted into the red oxide. The latter oxide, in combination with the gallic acid, assumes a black colour, and with prussic acid a blue colour. In these cases, where there is a change in the composition of the body, accompanied with a change of colour, the cause of this change is obvious; because every change in the composition of a body produces some change in the affinity, and therefore in the size, density, and figure of the particles; and it is not improbable in all of these circumstances together. But if the affinity of any body Of Colours, dy for other substances has undergone a change, it is natural to suppose that its affinity for light is also in some degree altered. This, however, although it happens in many instances, is not constant and uniform; because it may happen, that the changes in the size, density, or figure of the particles of the body, are such as to render it capable of combining with, or reflecting, the same rays of light as before it suffered any chemical change. Thus it must appear, that in most cases, the permanency of the colours of bodies will depend greatly on the permanency of their composition, and on the force of the affinities which they have for other bodies, to whose action they may be exposed.

44. In the ingenious experiments of Mr Delaval, which we have already alluded to, he has shown that coloured matters do not reflect any light. "Reflective media," (he observes), "act indiscriminately on all the different rays. It does not appear from the optical phenomena which have hitherto been observed, that nature affords any kind of matter endowed with a power of reflecting one sort of rays more copiously than the other sorts; consequently no reflective substances are capable of separating the differently refrangible rays, and thereby producing colours. There are several experiments and observations in Sir Isaac Newton's optics, from which it might have been inferred, that coloured light is not reflected from coloured matter, but from white or colourless matter only. Although that great philosopher supposes that all coloured bodies reflect the rays of their own colours more copiously than the rest, yet he observes that they do not reflect the light of their own colours so copiously as white bodies do. If red lead, for instance, and white paper, be placed in the red light of the coloured spectrum, made in a dark chamber by the refraction of a prism, the paper will appear more lucid than the red lead, and therefore reflects the red-making rays more copiously than red lead doth.*

* Optics, book i. part ii. prop. 5.

"If it be supposed that the red particles of the minimum reflect the red rays more strongly than the rest, what reason can be assigned why minimum should not exhibit the red rays as vividly as white paper, which acts indifferently on all the rays? But if it be considered that in opaque coloured bodies, the rays which are reflected from white reflective matter pass back through the transparent coloured media with which the reflective matter is covered, it will evidently appear why the coloured light reflected from white paper is more copious and bright than that which is exhibited by red lead.

"A considerable part of the incident light is lost in passing through transparent coloured media; therefore the light reflected immediately from the white paper, must be more copious and lucid than that which has undergone a diminution in its passage to and from the reflective particles of the opaque coloured body, through the transparent coloured medium.

"When a small portion of colouring matter is mixed with a colourless medium, the mass appears tinged with colour; but when a great quantity of colouring matter is added, the mass exhibits no colour, but appears black; therefore, to attribute to colouring matter a reflective power, is to advance an inexplicable and contradictory proposition; for it is asserting that in proportion as more reflective colouring matter is opposed to the incident light, less colour is reflected; and that when the quantity of colouring matter is very great, no colour at all is reflected, but blackness is thereby produced."

45. "From these arguments it might have been proved, shewn, that the reflective power does not exist in colouring matter, but in opaque white substances only. Nevertheless, in this disquisition, I have not entirely relied on arguments drawn from a few known and obvious appearances, but have endeavoured, by numerous experiments, to ascertain the cause of the colours of natural, as well as artificial bodies, and the manner in which they are produced.

46. "M. Euler observed, that the colours of bodies are not produced by reflection. He supposes that the coloured rays are emitted by the colorific particles. This hypothesis, however, is not agreeable to experiment; for as the colouring matter acts upon light by transmission only, it is evident that bodies do not appear coloured, either by reflecting or emitting the rays. I have not attended to any other hypotheses which are unsupported by experiments. Sir Isaac Newton, and I believe all later philosophers, except M. Euler, have attributed to colouring matter a reflective power; and the artists whose works depend upon the preparation and use of colouring materials, seem in general to have adopted the same theory. As an instance of this agreement, I have cited, from M. Hellot, one of the most skilful and intelligent authors, who have treated of the art of dyeing, a passage which comprises his opinion respecting the action of the tinting particles on the rays of light (A). All the other writers on the same subject, appear to agree in that established opinion; but they seem rather to have yielded to the authority of Sir Isaac Newton and other theorists, than to have appealed to the operations of their own art, from which the real cause and origin of colours is obviously deducible."†

47. "The art of dyeing consists principally in co. Mem. ii. revering white substances, from which light is strongly reflected, with transparent coloured media, which, according to their several colours, transmit more or less copiously the several rays reflected from the white substances.

(A) The passage from Hellot is the following. "At present we know only of two plants which afford a blue colour after their preparation. The one is the jatatis or glaftum, otherwise called paflet or wood. In the preparation of these plants, the fermentation is continued till the putrefactive process of all the parts of the plant, the root excepted, has been induced; consequently there takes place a separation of all their principles, with a new combination and arrangement of these same principles, from which results an assemblage of particles greatly divided, which being applied to any substance, reflect the light in a very different manner from what they did when those particles were combined with the other parts of the plant, previous to fermentation." Art de la Teinture des Laines, p. 117. Of Colours, &c. The transparent coloured media themselves reflect no light; and it is evident that, if they yielded their colours by reflecting instead of transmitting the rays, the whiteness or colour of the ground on which they are applied would not anywise alter or affect the colours which they exhibit. Such an erroneous conception of the principles of the art cannot fail greatly to obstruct its progress and improvement.

All colouring matter is black when viewed by incident light, and all substances inclined to blackness, in proportion as they are copiously stored with tingling particles.

48. As a farther illustration of this subject, we shall make another extract from the same author. "For the purpose," he observes, "of procuring matters made up of colouring particles, I reduced several transparent coloured liquors to a solid consistence by evaporation. When a gentle heat is employed in this operation, the colouring matter, which is thus concentrated, remains unimpaired, and capable of again imparting its colour unaltered, to other liquors. In this state the colouring particles reflect no colour; and as no light is transmitted through them, they are black. Among the liquors which I evaporated, were the tinctures and infusions of the colouring particles of red, purple, blue, and yellow flowers; of logwood, Brazilwood, saffron, turmeric, red fandiers, alkanet, lap-green, kermes, and other transparent coloured liquors, which are capable of being reduced to a solid consistence, without undergoing such changes during their evaporation, as to render them opaque."

49. "White paper and linen may be tinged by dipping them in the infusions of flowers of different colours; and by spreading upon those white grounds the expressed juices of such flowers, their colours may be communicated to the paper and the linen. These means of tinging are somewhat similar to the application of vegetable dyes to linen, and of transparent water colours to paper, many of which consist of the colouring matter of plants, such as indigo, litmus, gamboge, &c.

50. The consideration of these white substances affords much insight into the manner in which the natural colours of vegetables are produced. When the colouring matter of plants is extracted from them, the solid fibrous parts, thus divested of their covering, display that whiteness which is their distinguishing character. White paper and linen are formed of such fibrous vegetable matter, which is bleached by dissolving and detaching the heterogeneous coloured particles. When these are dyed or painted with vegetable colours, it is evident that they do not differ in their manner of acting on the rays of light, from natural vegetable bodies, both yielding their colours, by transmitting through the transparent coloured matter the light which is reflected from the white ground; for it appears, that no reflective power resides in any of their component parts, except in their white matter only."

51. Thus then it appears, that the colouring particles with which stuffs are dyed, being transparent, the reflected light must proceed entirely from the fibres of the cloth or stuff which are covered with the transparent colouring matter. If the stuff be already of a black colour, no other colour can be communicated to it; because it has not the power of reflecting any colour, and therefore it cannot transmit any. And if the stuff were of a red, blue, or yellow colour, it could not be dyed of any other colour excepting black; because the red, blue, or yellow rays only being reflected, no other rays could be transmitted. But these observations will strictly apply only when the whole of the surface of the cloth is of one uniform colour. Stuff to be dyed must be of a pure white colour before it is dyed, especially be pure white, when it is to be dyed any bright colour; for then the rays are copiously reflected; so that any colour may be given by combining with it any colouring matter which has the power of transmitting only particular rays.

52. As it is by the force of affinity that the colouring matter enters into combination with the stuffs which are dyed, that this chemical action be complete, it is necessary that the matter be in a state of minute division. No permanent colour could be produced by merely covering the surface of the fibres of the stuffs with the colouring substance; for unless it adhere so strongly that it cannot be separated by mechanical action, or by means of any of the processes to which dyed stuffs must be subjected, it must appear to be of little value, and the object in view is not obtained. To allow the chemical action to take place between the colouring matter and the stuffs, the former is dissolved in some liquid, for which it has a weaker attraction than that for the stuffs; so that when they are immersed in the solution, the colouring matter, in consequence of the stronger attraction which it has for the stuffs than for the solvent, combines with them, and thus they are dyed; and the facility with which this combination takes place, must obviously depend on the affinity between the colouring matter and the liquid holding it in solution, and the affinity between the cloth and the colouring matter. When these two affinities balance each other, no change takes place; but when the affinity between the stuff and the colouring matter prevails, the combination is effected, and the process proceeds more or less rapidly according to the force of this affinity.

53. Coloured bodies are compounds; and several substances enter into their composition. In all coloured bodies some of the component parts have a strong affinity for oxygen, which they attract from the atmosphere. The permanency of a colour consists in its power of resisting the action of all substances to which it is exposed. This power varies greatly according to the nature of the colour and the kind of stuff. The durability of the same colours on animal and vegetable matters is very different. But before the colour of a body can be permanent, all its component parts must be combined together by such strong affinities, that the substances which come in contact with them shall not be able to unite with any of these parts, and thus form a new compound. Should such a decomposition take place, the colour of the body cannot be permanent; and if the decomposition be suddenly effected, the colour is immediately destroyed. If the new combination proceeds slowly, the decay of the colour is also slow and gradual.

54. The combination of oxygen with some of the component parts of a coloured body, is one of the principal causes of the change of colours. The action of oxygen produces a change of colour.