478 Mr. De la Rue on Cochineal. 



D. Half a pound of cochineal was boiled with five pints of 

 alcohol, spec. grav. '830. The filtered tincture deposited on 

 cooling a granular precipitate, consisting chiefly of fatty 

 matter retaining a portion of colouring matter; on concen- 

 trating the tincture by distillation a further quantity was 

 deposited, which was filtered off; the filtrate was evaporated 

 to dryness in vacuo, when after eight weeks a gummy resi- 

 due was obtained. This mass dissolved with great difficulty 

 in a large quantity of absolute alcohol, a red flocculent 

 substance consisting chiefly of nitrogenous matter remaining 

 undissolved. The alcoholic solution filtered off" from this 

 deposit, concentrated by distillation and finally evaporated in 

 vacuo over sulphuric acid, dried to a tenacious semi-solid 

 mass, covered with a colourless oily fluid, and containing cry- 

 stalline particles of a solid fat. After removal of the fats by 

 means of sether, this mass was digested in water at 38° C, 

 which partly dissolved it with a fine red colour, leaving a 

 brown mass of resinous aspect behind, more of which de- 

 posited on the cooling of the coloured liquid ; the decoction 

 was now evaporated to the consistence of a syrup, and finally 

 dried in vacuo over sulphuric acid. 



These are all the processes employed to extract the colour- 

 ing matter from the cochineal; 1 may here remark, before 

 entering on the details of its further purification, that I ob- 

 tained other substances on evaporating the mother-liquors 

 from which the colouring matter had been separated by lead 

 salts, which will be hereafter described. 



Purification of the Carminic Acid. — In my first attempts 

 to purify the colouring matter I proceeded in the following 

 way : — An aqueous solution of the crude carminic acid (A) 

 was precipitated with acetate of lead, the precipitate of car- 

 minate of lead well-washed and decomposed by hydrosulphu- 

 ric acid ; the red supernatant liquid was first concentrated on 

 the water-bath and finally dried in vacuo; a highly hygro- 

 scopic purple residue was thus obtained. 



I could not, by whatever means I adopted, effect the deco- 

 lorization of the colouring principle. In several attempts I 

 heated the solution for some hours to 100° C, keeping up a 

 continuous current of hydrosulphuric acid, and in other ex- 

 periments a stream was made to pass for several days through 

 the disengaged colouring matter, but without the slightest 

 change in its aspect. From these experiments, made with 

 the greatest care and at several periods, I am led to the same 

 conclusion as Arppe, that Preisser must have been mistaken 

 in his results, and I regret that I cannot throw any light on 

 the probable cause of his error. 



