CARMINIC ACID. 95 



alcoholic solution of carminic acid, or of carminate of am- 

 monia. Such solutions stain, but stain weakly and diffusely. 



145. Carminic Acid occurs as a purple-brown mass, easily 

 soluble both in water and in alcohol. It is (according to 

 NIETZKI, Chemie der organischen Farlstojfe, Berlin, 1889, 

 pp. 231 234) a weak (LIEBERMANN says a strong) dibasic acid, 

 which forms soluble salts with the alkaline metals, insoluble 

 violet-coloured ones with the earthy and heavy metals. Very 

 little is known concerning the chemical nature of these salts. 



The alumina salt (carminate of alumina) may be obtained 

 by precipitating a solution of carminic acid or of carminate 

 of ammonia by means of acetate of alumina. It has the 

 remarkable property of being soluble not only in acids and 

 acid salts, such as alum, but also in alkalies and alkaline salts, 

 such as borax, provided that only water or weak alcohol be 

 employed as the menstruum. It is also precipitated from the 

 above-named solutions by chloride of aluminium, but only in 

 part ; whilst if alum be taken no precipitate is produced, the 

 carminate of alumina remaining in solution. Hence the 

 composition of the staining fluid given below under the name 

 of Carmalum. 



When chloride of aluminium is taken, a precipitate of 

 carminate of alumina is formed, as stated above. But this 

 precipitate will redissolve if more chloride of aluminium be 

 cautiously added. This gives the staining fluid described in 

 152, which may be convenient in cases in which it is not 

 desirable to work with a fluid containing alum. 



Both of these solutions stain in a violet tone, something like 

 alum-carmine. A redder tone may be obtained by adding 

 calcium chloride to the Carmalum solution. But this is not 

 advisable, for calcium chloride added to Carmalum precipi- 

 tates the solution with formation of gypsum. Of course, this 

 does not occur with the aluminium chloride solution; but for 

 other reasons the addition does not give satisfactory results 

 with the chloride of aluminium solution mentioned above. 

 But it does gives good results when combined with an alcoholic 

 chloride of aluminium solution, and thus -solves at once the 

 problem of obtaining a red stain and an alcoholic staining 

 fluid. This is described below under the name of Paracarmine. 



If the foregoing explanations of the rationale of carmine 



