4S4 Mr. De la Rue on Cochineal. 



doubled, and that the composition of carminic acid is ex- 

 pressed by the formula, 



C28 Hi4 O16, 

 as may be seen from the following table containing the com- 



})arison of the theoretical per-centages with the mean of ana- 

 yses IX. and X. 



Theory. Experiment. 



C29 . 168 54-19 5413 



Hi4 . 14 4-52 4-62 



Oje- . 128 41-29 41-25 



310 100-00 100-00 



From the foregoing experiments, it seems that the best 

 method of obtaining pure carminic acid is to precipitate 

 the aqueous decoction by acetate of lead ; to decompose the 

 washed carminate of lead by hydrosulphuric acid, and to 

 throw down the carminic acid once more by acetate of lead, 

 previously mixed with acetic acid ; to decompose the carmi- 

 nate of lead by hydrosulphuric acid ; to evaporate to dryness 

 and redissolve the carminic acid in absolute alcohol ; then to 

 digest the alcoholic tincture with carminate of lead ; and lastly, 

 to precipitate the trace of nitrogenous matter by aether, the 

 pure carminic acid is obtained from the filtrate. 



As thus prepared, carminic acid has the following proper- 

 ties. It is a purple brown friable mass, transparent when 

 viewed by the microscope, and pulverizing to a fine red 

 powder ; soluble in water and alcohol in all proportions, very 

 slightly soluble in aither, which does not however precipitate 

 it from its alcohoHc solution if free from nitrogenous matter. 

 It is soluble without decomposition in concentrated hydro- 

 chloric and sulphuric acids. It is decomposed by chlorine, 

 iodine and bromine, which change its colour to yellow, and 

 the latter on warming or by standing gives a yellow precipi- 

 tate soluble in alcohol. Nitric acid decomposes it even if 

 highly diluted : I shall have occasion to refer to this decom- 

 position presently. It bears a temperature of 136° C. with- 

 out decomposition ; on gradually increasing the temperature 

 a quantity of an acid liquor is produced, and at a red heat it 

 intumesces and gives oft" a small quantity of red fumes, which 

 condense : it gives no trace of oily matter. 



The aqueous solution has a feeble acid reaction ; it does not 

 absorb oxygen. A volume of this gas contained in a tube 

 with carminic acid over mercury did not change by absorption 

 after exposure for several months. The fixed alkalies and 

 ammonia give no precipitate in the aqueous solution, merely 

 changing its colour to purple j in the alcoholic tincture they 



