Mr. De la Rue on Cochineal. 477 



tion of the colouring matter, capital letters will be used to 

 designate the various preparations. 



A. About 3 lbs. of ground cochineal (technically known as 

 shelly black) was introduced into 15 gallons of boiling distilled 

 water, and the mixture maintained at that temperature for 

 twenty minutes ; the decoction, strained through a sieve, was 

 allowed to subside for a quarter of an hour and then decanted 

 off; whilst still hot the transparent liquid was mixed with 

 basic nitrate of lead, added with caution to avoid excess ; a 

 fine purple lake was thus obtained, the supernatant liquor 

 retaining only a pale buff tinge. After decantation of the 

 supernatant liquor, the lake was thrown on a cloth filter and 

 washed with distilled water until the filtrate gave but a slight 

 opalescence with chloride of mercury, which was found to be 

 a test for the presence of nitrogenous matter. The lead lake 

 was then suspended in distilled water and treated with a co- 

 pious stream of hj'^drosulphuric acid, when a precipitate of 

 sulphuret of lead and a deep red supernatant liquid was ob- 

 tained ; on stirring the liquid this colour almost disappeared, 

 the colouring matter being evidently absorbed by the sul- 

 phuret, agreeing perfectly with Arppe's observation. A fresh 

 stream of gas reproduced the colour, which was again absorbed 

 on stirring; after continuous treatment with hydrosulphuric 

 acid, the lead lake being completely decomposed, the filtered 

 liquid was evaporated in a water-bath to a syrupy consistence, 

 and the evaporation finished as far as possible at a tempera- 

 ture of 38° C. The semi-solid substance thus obtained was 

 of a deep purple colour, had a strongly acid reaction, and 

 evolved the smell of burnt sugar, as noticed by Arppe. The 

 weight of this substance, which I call crude carminic acid, 

 was 3^ ozs., and 1 oz. more was obtained from the residue by 

 similar treatment. 



B. On repeating the same process the whole product was 

 lost. An excess of the basic nitrate having been employed, 

 the nitric acid set free by the hydrosulphuric acid caused a 

 violent decomposition, with evolution of nitrous fumes, as 

 soon as the carminic acid arrived at a pasty consistence ; this 

 agrees also with Arppe's experience. 



C. In this operation a decoction of cochineal, made in the 

 described manner, was precipitated with a solution of acetate 

 of lead acidulated with acetic acid (six parts by weight of 

 crystallized acetate, and one part of strong acetic acid). The 

 resulting lead lake, being very bulky, was washed by decan- 

 tation with boiling distilled water, collected on a filter, dried 

 in a current of warm air, and finely powdered; 17 ozs. of 

 crude cai'minate of lead were thus obtained. 



