476 Mr. De la Rue on Cochineal. 



nium and acids rendered it perceptible. He likewise tried 

 to obtain the colouring matter in a state of purity by pre- 

 cipitating with hydrated oxide of lead an aqueous decoction of 

 cochineal (previously purified from nitrogenous substances by 

 nitrate of silver). On treating the precipitate by hydrosul- 

 phuric acid, he obtained a red liquor of strongly acid reaction, 

 the acid of which Avas not derived from the lead salt : on eva- 

 poration it left a dark red mass, emitting the smell of burnt 

 sugar. Wishing to separate the acid, which he thought 

 contaminated the colouring matter, he prepared a strong 

 aqueous decoction of cochineal, and after separating the ni- 

 trogenous matter by means of nitrate of silver, filtering, and 

 then saturating by ammonia, and afterwards adding the 

 hydrated oxide of lead, he found that the supernatant am- 

 moniacal liquor, which was nearly colourless, yielded by 

 evaporation an acid liquid ; and on decomposing the lead lake 

 with hydrosulphuric acid, he obtained a liquid slightly 

 coloured (the colouring matter adhering to the sulphuret of 

 lead), which was likewise acid. From this he concluded that 

 the colouring matter had not up to that period been obtained 

 in a state of purity. 



Microscopic Examination of Living Cochineal. 



By the kindness of Sir James Clark, who furnished me with 

 specimens of the living insect, I have been enabled to examine 

 the physical characters of the colouring matter as it exists in 

 the organism of this little insect before it is subjected to the 

 process of drying for commerce. On examination by the mi- 

 croscope, the white dust which covers the insect and the ad- 

 jacent parts of the cactus leaves, on which it feeds, has all the 

 characters of an excrement ; it has a curled cylindrical form, 

 is of very uniform diameter and of a white colour. On re- 

 moving the powder with a little aether and piercing the side 

 of the little creature, a quantity of a purplish red fluid exudes, 

 which contains the colouring matter in minute granules as- 

 sembled round a colourless and larger nucleus, and these 

 groups float in a colourless fluid. It is evident from this, 

 that, whatever may be the function of the colouring matter, it 

 has a distinct and marked form, and does not pervade as a 

 mere tint the fluid portion of the insect. 



Separation of the Colouring Matter. 



It became evident from a few preliminary experiments that 

 the investigation would be greatly facilitated by the employ- 

 ment of a large quantity of material ; and as in the course of 

 the inquiry different methods were adopted for the prepara- 



