178. By means of Brazil wood, which, like almost all the other woods loaded with colour, gives a fading dye, I prepared a red which was much finer than the madder reds, and as bright as those made with kermes ; this red remained exposed to the air for the two last months of the year 1740, which were very rainy, and for the two first of 1741 ; notwithstanding the rain and bad weather, it stood, upon account of its peculiar preparation, which will be mentioned in the proper place, and was so far from losing, that it acquired body. Yet the same led which is so durable in the air, does not resist the trial by tartar. Would it then be proper to proscribe this colour, because that salt discharges it, or are the stuffs we employ for clothing intended to be boiled with tartar, alum, or soap ? I do not, however, mean to disapprove of the trial by proofs; they are useful because they are ready, but there are cases in which they ought not to serve as a ground for pronouncing a sentence of confis cation, especially when they cannot show that a colour which should have been dyed from materials for durably colours, has been dyed with those of a fading nature." 179. Upon the whole, then, it appears that the only sure test we can employ, is direct exposure to air and light, for those stuffs which are not to be leyed; and washing with alkalies and soap for such as are to be occasionally exposed to these substances. In order to try the colours of silk, it is generally thought sufficient to expose them to heat in acetous acid, or lemon juice ; and they are considered as good and permanent colours if they stand this test. And, indeed, when the woods or archil alone have been employed, the colours are made red by the, action of a vegetable acid; but if the solution of tin has been used to dye w ith these substances, the colour which has been prepared in an acid liquor, is not afterwards affected by vegetable acids, and in that ca.(' a colour may be reckoned good, which has been touch less czpensiwe, and w ill prove less durable. So that for silk also the only test we can rely upon with onfidence is exposure to the air and light.
OF 180. Though a knowledge of the structure of orga nized bodies, and of the proportions in which their ul timate principles arc combined together, can seldom, the aid of experiment, afford much information respecting their relations with other substances, we can not doubt that an intimate acquaintance with their me chanical form and chemical composition %vill throw some light on these relations, and point out the best means of investigating such as cannot be discovered by analogy alone. We shall therefoie endeavour to ascertain the mechanical and chemical constitution of the various kinds of stuffs, as a knowledge of these may assist us in discovering the cur umstances upon which their at tractions for colouring matters depend.
181. The stuffs usually submitted to the operation of dyeing, are wool, silk, cotton, and flax. The two first arc of animal origin; the two last are vegetable pro ducts. These substances disagree with one another in chemical composition, and arc characterised by those differences of constitution which usually distinguish the productions of the animal and vegetable kingdom. Ani mal substances differ from vegetable products, by con taining a considerable quantity of nitrogen, an element which exists but sparingly in vegetables; and also a larger proportion of hydrogen. This difference of che
mical constitution is rendered very obvious, when ani mal and vegetable products are subjected to distillation. The former yield a large quantity of ammonia, the latter seldom give out that substance, but frequently an acid. The former yield much oil, while, in many cases, vege table products do not yield the smallest quantity. Ilence, ,Inintal substances, during their combustion, afford a bright flame, though of short continuance, as it is soon xtinguished by the charcoal which is formed ; they, at the same time, emit a very pungent odour, owing to the extrication of ammonia, and of the empyreumatic oil, which makes its escape V. ithout being consumed. Noimal substances arc very liable to putrefaction, and during the ads ancemem of the process, evolve much ammonia ; while vegetable products suffer more slowly the Vinous or aeetous fermentation.
182. As substances arc most readily decomposed when the elements which enter into their composition arc numerous, animal matters are more liable than ve getables to be destroyed by the different agents to which they are exposed. The number of elementary princi ples which they contain, enlarges in an equal degree the sphere of their chemical affinity ; and hence they are also mot e disposal to combine with colouring matters. Thus the pure alkalies quickly destroy animal substan t es, because they combine with them, and lose their causticity, while they have little or no effect on vege tables. The nitric and sulphuric acids also act with considerable energy on animal substances; the former decomposes them, extricates the nitrogen, separates the fatty matter, and forms carbonic and oxalic acid ; the latter extricates the hydrogen, a small portion of nitro gen, and leaves a carbonaceous residuum.
183. Silk, though an animal product, partakes a good deal of a vegetable nature. It is less disposed to com bine with colouring matters than wool, and resists bet ter the action of acids and alkalies. Cotton and flax withstand the action of acids and alkalies more power fully than wool and silk, the former being destroyed with even by the nitric acid. These diffe rences arc chiefly owing to differences in chemical con stitution ; but they may be partly ascribed to differen ces in the conformation of their fibres and organic structure.
184. The different kinds of wool vary chiefly in the length and delicacy of the fibre. Its qualities depend partly upon the breed of the sheep from which the wool is taken, and partly upon the parts of the animal to which it adhered. The finest wool is brought from Spain ; though Berthollet affirms, on the authority of D'Aubenton, that it might be produced in France of a quality equally good, by a little attention on the part of the shepherd. Attempts have been made of late to introduce the Spanish breed of sheep into this country, with the view of rendering us independent of a supply of wool from abroad ; but it is probable that change of climate may affect the habits of the animal, and produce alterations on the fleece. It is \veil known that the na ture of the pasture has a considerable effect on the qua lity of wool. The manufacturer should be able to judge with accuracy respecting the fineness of it ; and, as simple inspection is insufficient for the purpose, the fibres should be closely examined with a microscope, and compared with those of other wools, selected as standards.