169. " When, therefore, we wish," continues he, " to compare together two or more colouring substances of the same nature, and to determine the relative quality and quantity of the colouring particles in each, we need only compare the quantities of the same oxygenated mu riatic acid required to produce the same degree of de terioration in an equal weight of each; the qualities of these substances, or the quantity of colouring particles they contain, are directly proportionate to the quantities of liquor required to produce the same effect on each ; but, in this experiment, it is necessary that the colour ing matter of each substance should have been dissolved in a proper liquor, and that all the circumstances in the process of comparison should be similar. If we wish, for example, to compare several kinds of indigo, we take an equal weight of each, powder them carefully, and put them into separate matresses, with eight times their weight of concentrated sulphuric acid : These arc to be kept for twenty-four hours, in a heat of from 30° to 40° (86° to 104° of Fahrenheit). Each solution is then to be diluted with a quantity of water, filtered, and the residue found on the filters to he collected, and ground in a glass mortar, and again digested with the addition of a little more sulphuric acid : the last solu tions are also diluted with equal quantities or water, filtered, and each added to its corresponding liquor. Finally, to each solution is added as much oxygenated muriatic acid as will discharge the colour, or rather bring them all to the same shade of yellow. The quali ties of the different kinds of indigo, are proportionate to the quantities of oxygenated muriatic acid required to destroy their colour. The proofs of such colouring par ticles as are soluble in water, are much more simple : it is only requisite, after having extracted them as much as possible, to mix oxygenated muriatic acid with equal bulks of the decoction, of the same weight of each sub stance, and to compare the quantity of acid required to bring them all to the same hue." 170. Though we are inclined to adopt the general principles of Berthollet's theory, and to admit, that the oxygen of the atmosphere and the oxymuriatic acid act upon colouring matters in a manner somewhat similar, we are not disposed to believe, with this eminent che mist, that the mode of action in the two cases is exactly alike, or, that the effects produced by the one afford a precise criterion of the nature of the operation of the other. Dr Bancroft has very justly remarked, that the properties of oxygen are greatly diversified by its union with other bodies ; and that, in selecting the oxymuria tic acid as an accurate test of the changes which colour ing matters suffer from exposure to the air, Berthollet entirely overlooked the action of the basis with which the oxygen is united; though there cannot be a doubt, that so powerful an agent as the muriatic acid must greatly modify the effects of the oxygen. Dr Bancroft mentions several experiments which add considerable weight to his objections, and at least prove, that the oxymuriatic acid cannot, in all cases, be considered as an accurate test of the durability of colour. He put into a small phial cuttings from three skains of cotton yarn, which had been dyed and sent to him by Chaptal. One of these had received the Turkey red, another the Nankeen buff from an oxide of iron, and the third a black from madder and galls, as he supposed, applied upon the basis of iron dissolved by the pyrolignous acid. 66 Upon these colours," says he, 66 I poured oxymuriatic acid, which had been prepared by 1\Ir Accum, and kept secluded from light. Its acidity was so slight as to be hardly perceptible to the taste, and, I believe, it might have been put into the eye without occasioning much pain. I found, however, that in less than two minutes the colour of the Turkey red was much impaired, and, in five, the yarn throughout the greater part of its sur face had become white, without passing through any intermediate colour; and, at the end of half an hour, but a very few sparks of red, less than a pin's head, were perceptible. The buff colour, at that time, was found to have acquired a little body, and the black to have lost a little, but without ceasing to be still a good black. At the same time, I put other cuttings of the same colours into another phial, and poured upon them undiluted aquafortis as prepared for the scarlet dyers ; and I found, that, in a single minute, the black, which had withstood the oxymuriatic acid, was changed to a buff colour, resulting solely from the ferruginous basis with which it had been dyed; and the Turkey red began to exhibit the appearance of a scarlet, inclining to the orange ; and this last (of a lively tint) became apparent ly its settled colour at the end of an hour, the buff, by acquiring more oxygen, was considerably raised. Here, then, was a very great diversity between the effect or the nitric and the oxymuriatic acids, in no degree accord ing or proportionate to their degrees of acidity ; that of the nitric acid being at least fifty, and perhaps one hun dred times greater than that of the oxymuriatic acid, (which being tasted at the time when its action upon the Turkey reel was strongest, and when, according to Davy's opinion, it must have already decomposed water, had not, to my taste, acquired any greater degree of acidity); and yet the former could only change the com plexion of the Turkey red to a bright orange, (probably by imparting oxygen to it,) whilst the latter (not, as I conceive, by any such, or other addition, but by a com plete decomposition,) had at once annihilated all the co lour (leaving the cotton yarn white) as fast and as far as the decomposition took place; and this without any in termediate tint, which would not have been the case, if the effect of the oxymuriatic acid had, as M. Berthollet supposes, resembled combustion. And, on the other hand, the black, on which the oxymuriatic acid could make but a very slight impression, was completely de stroyed (excepting the colour of its ferruginous basis) by the nitric acid." Dr Bancroft accordingly infers,, tbe cildattno (net led by the oxymuriatic acid nn coloi rin.• Mal ,, errs hot result from the abstrac tor of t s uNy but Irom some quality peculiar to itself; 1 out the combined a ;trey Of its constituent plus. or 1 not 1r Im tn. a u .1 of either separately, as has ;en supposed."
171. it is ohs IOW, 110W C Ver, that, in reasoning con , t in.,. • the t fleets of the hit, is and oxymoriatie acids, the Doctor has ( unfounded two things which arc perfectly (hstintt : the tick itv of (Lest. bodies, and their disposi tion m pall with their oxy gen. Even admitting that the entity of oxygen contained in acid bodies was propor tional to their aeidity, it would not follow, that the most in id substances would most readily impart their oxygen to other bodies. The nitrous oxide supports combustion ith a brilliancy little inferior to oxygen gas itself; the nitric oxide, which, with the same base, contains double the quantity of oxygen, so far from supporting combus tion, immediately extinguishes the greater number of burning bodies which are plunged into it. It cannot he inferred, therefore, that, because nitric acid possesses greater acidity than oxyinuriatic acid, the former must necessarily give out more oxygen than the latter, to the substances brought within the spheres of their activity. But though this argument is by no means conclusive with respect to the dissimilarity of circa pi oduced by atmospheric air and oxymuriatic acid, and though seve ral explanations might be offered to account for the dif ferent results obtained in the preceding experiments, in perfect consistency with Berthollet's theory, the follow ing facts, which Dr Bancroft has stated, certainly tend to prove, that whatever analogy may hold between the action of common air and oxylnuriatic acid, the latter cannot be regarded, in all cases, as a measure or indica tion of what colours would suffer by exposure to sun and air.
172. Ile put into an empty glass-stopped phial, the following colours upon separate bits of muslin, viz.
" 1st, A fast madder-red, dyed topically by an eminent calico printer, upon a basis, from acetate of alumine applied by the block.
A fast yellow, dyed from weld upon the same basis, by the same calico printer.
" 3d, A fast yellow, dyed upon the same basis, from quercitron bark.
"4th, A fine durable purple, produced by the colour ing matter of the buccinum lapillus.
" 5th, A logwood purple, produced by' mixing ‘vitli strong decoction of that wood as much muriate of till as rendered the former slightly acid, and, after thickening the mixture with gum-arabic, applying it in spots to muslin; which, after being properly dried, was washed tt ith soap and water.
"6th, A full bright yellow, produced from a similar decoction of the quercitron bark rendered slightly acid by all admixture of nitro-muriate of tin, made with two parts of nitric to one part of moriatic acid; gummed and topically applied in the same manlier as the log wood purple, and in like manner dried, and afterwards washed.
" 7th, A similar yellow', made from the quercitron bark, only substituting mu•io•sulphatc of tin for the Lit ro-muriate." 173. " Upon these colours," says Dr Bancroft, " I poured oxymuriatic acid, with which 'Air Accum had recc tidy supplied me, (and which I had kept secluded from the light) until the phial was full; after which, in less than two minutes, I found that the bits of muslin, with the madder, weld, and quercitron colours, dyed upon the aIaltitlous basis, were become perfectly IV bile, by a complete extinction of their several colours. Whilst the logwood purple, that from the bucciouni, and the quercitron yellows, with solutions of till, were not ap parently changed. But, in about live minutes, the log wood purple appeared to be losing body, as did the quercitron yellows soon after ; and a similar effect soon became evident in the shell purple. hi about fifteen minutes from the time when these colours were im mersed in the oxymuriatic acid, the logwood purple had nearly disappeared ; and this was the case of the quer citron yellows in about three minutes afterwards, and of the shell purple abort two minutes later; excepting that a part of the latter, as well as a part of one or the yellows given with tin, had each preserved a portion of colour, by having been protected by the bits of muslin from the sun's rays, which, as the sky was clear, bad had free access to the phial containing them at the win dow' where this experiment was made; a fact which manifested the influence of solar light in promoting the destructive action of the oxymuriatic acid on the colours in question. It is here to be recollected," continues he, " that the three lirst-mentioned colours, dyed upon the aluminous basis, would have resisted the action of sun and air for two or three months, and the madder for a much lodger time; and yet they were completely de stroyed in an eighth part of the time which was required to destroy the logwood purple, and the yellows with tin; neither of which could have been exposed to the sun and air for a single week without becoming of a faded brown. It is also worthy of observation, that the Tyrian or shell purple was destroyed by the oxymuriatic acid almost as soon as the logwood purple and quercitron yellow's last mentioned, though it would have resisted the sun and air probably fifty times longer than either of them." These experiments certainly shew, that what ever similarity may subsist between the effects of oxy muriatic acid and atmospheric air on colouring matters, the former cannot be regarded as the very accurate test of the durability of colours, which Beithollet was dis posed to represent it. Indeed, the only sure mode of de termining the durability of colours, appears to be direct exposure to air, sun, water, and the other agents to which dyed stuffs arc most frequently. exposed. We shall therefore subjoin the observations of Ilellot on this subject, together with his account of the methods em ployed by Dufay, which laid the foundation of the regn lations adopted by the French government respecting fading and durable colours : " The late Mr Dufay," say's " who had been selected by government as a person whose labours might greatly improve the art of dyeing, made experiments on the subject, by dyeing wool of all colours, and even with dyes of all sorts, used either for durable or fading colours; he even sent to the different provinces 'or such as are not employed at Paris; and, finally, he collected most of the different substances which he Conceived might be employed in the art, and tried a very great number of them, investigating their good or bad qualities, without paying any regard to the prejudices of the dyers.