DYEING. 



233 



Oxymuria- 

 tic acid used 

 as a test of 

 the durabi- 

 lity of co- 

 lour. 



dominance of this charcoal, with its supposed naturally 

 black colour." 



168. As a practical inference from his theory, Ber- 

 thollet has proposed to employ the oxymuriatic acid for 

 determining the durability of colours : " When we 

 wish to examine a colour," says he, " we need only 

 put a pattern of the stuff into oxygenated muriatic acid, 

 along with a pattern of the same colour which has been 

 properly dyed ; the comparative power of resisting its 

 action, shewn by the colours of the two patterns, be- 

 comes the measure of their goodness ; but as this li- 

 quor has a very strong action on the colouring parti- 

 cles, it must be employed in a very dilute state. This 

 proof has also the advantage of shewing us very nearly 

 the shades and changes through which a stuff must 

 pass, when it comes to be acted upon by the air. Still, 

 however," he adds, with much philosophic candour, 

 " were it necessary to examine a colour juridically, I 

 should not venture to rely entirely upon the action of 

 the oxygenated muriatic acid, and would only decide 

 with confidence, from the results afforded by exposure 

 to the air." He afterwards asserts, however, that the 

 oxygenated muriatic acid seems to furnish a scale for 

 the comparison of colours as completely as could be 

 desired ; because, when we compare indigo with indigo, 

 the nature of the colouring particles is the same, or 

 nearly the same, and no foreign affinity can interfere, 

 to prevent the action of the oxygenated muriatic acid 

 from determining the proportional quantity of colour- 

 ing matter. 



169- " When, therefore, we wish," continues lie, " 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 oxygena- 

 ted muriatic acid required to produce the tame degree 

 of deterioration in an equal weight of each ; the quali- 

 ties of these substances, or the quantity of colouring 

 particles tliey 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 

 tin- colouring matter of each substance should have been 

 <li>-olved in a proper liquor, and that all the circum- 

 stances 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 matresscs, with 

 eight times their weight of concentrated sulphuric 

 acid : These are to be kept for twenty-four hours, in a 

 heat of from :iO to 40 (8fi to 101 J of Fahrenheit). 

 I'.ach solution is then to be diluted with a quantity of 

 water, filtered, and the residue found on the filters to 

 be et.lleeted, and ground in a glass mortar, and again 

 '(1 with the addition of a little more sulphuric 

 acid : the last solutions are also diluted with equal quan- 

 <if watiT, filtered, and each added to its corre- 

 s|xmding liquor. Finally, to each solution is added as 

 much oxygenated muriatic acid as will discharge tile 

 colour, or rather bring them all to the same shade of 

 yellow. The qualities of the different kinds of indigo, 

 are proportionate to the quantities of oxygenated mu- 

 riatic- ;,c-id required to destroy their colour. The proof's 

 of such colouring particles as are soluble in water, are 

 much more simple: it is only requisite, after having 

 extracted them as much as jx>ssible, to mix oxygenated 

 muriatic acid with equal bulks of the decoction, of the 

 ame weight of each substance, and to compare the 

 quantity of acid required to bring them all to the same 

 line." 



v>r. viu. PART I. 



170. Though we are inclined to adopt the general Durability 

 principles of Berthollet's theory, and to admit, that the ^j llour 

 oxygen of the atmosphere and the oxymuriatic acid """"V~ 

 act upon colouring matters in a manner somewhat si- Objections 

 milar, we are not disposed to believe, with this eminent to this test, 

 chemist, that the mode of action in the two cases is ex- 

 actly alike, or, that the effects produced by the one af- 

 ford 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 oxy- 

 muriatic acid as an accurate test of the changes which 

 colouring matters suffer from exposure to the air, Ber- 

 thollet 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 Fx rf 

 considerable weight to his objections, and at least mentTof T> 

 prove, that the oxymuriatic acid cannot, in all cases, Banci. ti. 

 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 dis- 

 solved by the pyrolignous acid. " Upon these colours," 

 says he, " I poured oxymuriatic acid, which had been 

 prepared by Mr 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 through- 

 out the greater part of its surface 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 tune, 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 a* 

 prepared for the scarlet dyers ; and I found, that, in a 

 single minute, the black, which hud withstood the oxy- 

 muriatic 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 appear- 

 ance of a scarlet, inclining to the orange ; and this last 

 (of a lively tint) became apparently its settled colour 

 at the end of an hour, when the buff, by acquiring 

 more oxygen, was considerably raised. Here, then, 

 was a very great diversity l>ctwecii the effect of the 

 nitric and the oxyimtriatic acids, in no degree accord- 

 ing or proportionate to their degrees of acidity; that 

 of the nitric aeid being at least fifty, and perhaps one 

 hundred times greater than that of the oxymuriatic 

 acid, (which being tasted at the time when its action 

 upon the Turkey red was strongest, and when, accord- 

 ing to Davy's opinion, it must have already decompo- 

 sed water, had not, to my taste, acquired any greater 

 degree of acidity); and yet the former could only 

 cliange the complexion 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 ad- 

 dition, but by a complete decomposition,) had at once 

 annihilated all the colour (leaving the cotton yarn 

 white) as i'^t and as far as the decomposition took 

 2 e 



