472 SUMMAKY OF CUERENT RESEARCHES RELATING TO 



mass, fusible and soluble in all proportions in water and in alcohol. 

 Sulphuric and hydrochloric acid dissolve it without alteration. It 

 bears a heat of 136° 0. without decomposition." Earlier still Watts 

 (Diet, Chem., i. 1863, p. 804) says:—" The fine red pigment known in 

 commerce as carmine is prepared by treating a solution of cochineal 

 with cream of tartar, alum, or acid oxalate of potassium. The fatty and 

 albuminous matters then coagulate and carry down the colouring 

 matter with them." Now in preparing most carmine solutions this 

 precipitation takes place, and the carmine, having greater cohesive (not 

 chemical) affinity for impurities of animal origin than for alcohol, its 

 solution is not readily accomplished by that medium, nor indeed by 

 water. In preparing carmine solution for histological purposes by 

 some of the published recipes, more than one-half of the colouring 

 matter of the carmine is lost in the refuse left upon the filter 

 paper. 



There are two ways commonly in use for preparing carminic acid. 

 The first mode is that of De la Eue, which Watts (Diet. Chem., i. 1863, 

 p. 804) gives as follows : — " To separate carminic acid, cochineal is 

 exhausted with boiling water ; the extract is precipitated by subacetate 

 of lead slightly acidulated, care being taken not to add the lead-solu- 

 tion in excess ; the precipitate is washed with distilled water till the 

 wash-water no longer gives a precipitate with a solution of mercuric 

 chloride, then decomposed by sulphuretted hydrogen ; the filtrate is 

 evaporated to a syrupy consistence and dried over the water-bath ; and 

 the dark purple product thus obtained is treated with alcohol, which 

 extracts the carminic acid." The second mode is that of C. Schaller and 

 is given by Watts (Diet. Chem., 1st suppl., 1872, p. 413) as follows : — 

 " Schaller prepares this acid by precipitating the aqueous extract of 

 cochineal with neutral lead acetate slightly acidulated with acetic acid ; 

 decomposing the washed precipitate with sulphuric acid ; again pre- 

 cipitating the filtrate with lead acetate, and decomposing the precipi- 

 tate with hydrogen sulphide. The filtered solution is evaporated to 

 dryness ; the residue dissolved in absolute alcohol ; the crystalline 

 nodules of carminic acid obtained on leaving this solution to evaporate 

 are freed from a yellow substance by washing with cold water, which 

 dissolves only the carminic acid ; and the residue left on evaporating 

 the aqueous solution is recrystallized from absolute alcohol or from 

 ether." 



Schaller's mode of preparation gives purer carminic acid than De 

 la Eue's, but either kind is sufficiently pure for histological purposes. 

 The precipitation by lead acetate and the dissolving in alcohol free the 

 carminic acid from animal impurities, and the consequence is a purer 

 form of pigment than can be extracted by any process hitherto 

 employed for the preparation of carmine for histological purposes. 



It is unnecessary to explain to naturalists the advantages of alco- 

 holic solutions of carmine over aqueous ones. The alcoholic solution 

 colours preparations much quicker than the aqueous solution does ; 

 for colouring sections, the author employs a solution of • 25 gr. car- 

 minic acid to 100 gr. of 80 per cent, alcohol, and leaves sections in the 



