COCHINEAL 877 



products. They were decomposed by the action of heat, with the diffusion at first of a 

 very strong smell of burning animal matter, and then of sulphur. A white powder 

 remained, amounting to about one-tenth of the matter employed, and which was found 

 to be alumina. Other quantities of carmine were treated with a solution of caustic 

 potash which completely dissolved them, with the exception of a fine red powder, 

 not acted on by potash and concentrated acid, and which was recognised to bo red 

 sulphurot of mercury or vermilion. This matter, evidently foreign to the carmine, 

 appears to have been added, in order to increase its weight. 



The preceding observations and experiments seem calculated to throw some light 

 on the art of dyeing scarlet and crimson. The former is effected by employing a 

 cochineal bath, to which there have been added, in determinate proportions, acidulous 

 tartrate of potash and nitro-muriatic deutoxide of tin. The effect of these two salts 

 is now well known. The former, in consequence of its excess of acid, tends to 

 redden the colour, and to precipitate it along with the animal matter ; the latter acts 

 in the same manner, at first by its excess of acid, then by the oxide of tin which 

 falls down also with the carmine and animal matter and is fixed on the wool, with 

 which it has of itself a strong tendency to combine. MM. Pelletier and Caventou 

 remark, that ' to obtain a beautiful shade, the muriate of tin ought to be entirely at 

 the maximum of oxidisement ; and it is in reality in this state that it must exist in the 

 solution of tin prepared according to the proportions prescribed in M. Berthollet's 

 treatise on dyeing.' 



We hence see why, in dyeing scarlet, the employment of alum is carefully avoided, 

 as this salt tends to convert the shade to a crimson. The presence of an alkali would 

 seem less to bo feared. The alkali would occasion, no doubt, a crimson-coloured 

 bath ; but it would be easy in this case to restore the colour, by using a large quantity 

 of tartar. We should, therefore, procure the advantage of having a bath better 

 charged with colouring matter and animal substance. It is for experience on the 

 large scale to determine this point. As to the earthy salts, they must be carefully 

 avoided. 



To obtain crimson, it is sufficient, as we know, to add alum to the cochineal bath, 

 or to boil the scarlet cloth in alum water. It is also proper to diminish the dose of 

 the salt of tin, since it is found to counteract the action of the alum. 



The alkalis ought to be rejected as a means of changing scarlet to crimson. In 

 fact, crimsons by this process cannot be permanent colours, as they pass into red by 

 the action of acids. 



According to M. Von Grotthuss, carmine may be deprived of its golden shade by 

 ammonia, and subsequent treatment with acetic acid and alcohol. Since this fact 

 was made known, M. Herschel, colour-maker at Halle, has prepared a most beautiful 

 carmine. 



The officers of Her Majesty's Customs detected some time since a system of adul- 

 terating cochineal, which had been practised for many years upon a prodigious scale 

 by a mercantile house in London. Dr. Ure stated that he had analysed about 100 

 samples of such cochineal, from which it appears that the genuine article is moistened 

 with gum-water, agitated in a box or leather bag, first, with sulphate of baryta in 

 fine powder, afterwards with bone or ivory black, to give it the appearance of negro, 

 cochineal, and then dried. By this means about 12 per cent, of worthless heavy spar 

 is sold at the price of cochineal, to the enrichment of the sophisticators, and the dis- 

 grace and injury of British trade and manufactures. 



The specific gravity of genuine cochineal is 1'25 ; that of the cochineal loaded with 

 the barytic sulphate, 1-35. This was taken in oil of turpentine, and reduced to water 

 as unity, because the waxy fat of the insects prevents the intimate contact of tho 

 latter liquid with them, and the ready expulsion of air from their wrinkled surface. 

 They are not all acted upon by the oil, but are rapidly altered by water, especially 

 when they have been gummed and barytified. 



Humboldt states that so long ago as the year 1736, there was imported into Europe 

 from South America, cochineal to the value of 15 millions of francs. Its high price 

 had for a long time induced dyers to look out for cheaper substitutes in dyeing red ; 

 and since science has introduced so many improvements in tinctorial processes, both 

 madder and lac have been made to supersede cochineal to a very great extent. 



In order to ascertain the value of cochineal for dyeing we must have recourse to 

 comparative experiments. We are indebted to MM. Eobiquet and Anthon for two 

 methods of determining the quality of cochineals, according to the quantity of carmine 

 they contain. The process of M. Kobiquet consists in decolourising equal volumes of 

 decoction in different cochineals by chlorine. By using a graduated tube, the quality 

 of the cochineal is judged of by tho quantity of chlorine employed for decolourising 

 the decoction. The process of M. Anthon is founded on the property which the 

 hydrate of alumina possesses of precipitating the carmine from the decoction so as to 



