190 Dr. Schunck on the Constitution of 



sacrificing somewhat of the minute agreement usually expected 

 between experiment and theory. Those conversant with this 

 species of research will indeed expect some discrepancies between 

 the calculated and experimental composition of substances, a 

 sufficient quantity of which for the purposes of analysis can sel- 

 dom be obtained without a great expenditure of time and labour. 



During my examination of alizarine, I discovered that this 

 substance is converted by the action of nitric acid into a crystal- 

 lized acid, which I called alizaric acid, and to which I attributed 

 the formula C 14 H 5 O 7 . MM. Laurent and Gerhardt have proved 

 that this acid is identical with Laurent's phtalic acid, and I am 

 obliged to them for the correction; but when M. Laurent pro- 

 ceeds to say, that, having made this discovery, they held in their 

 hands the key to all the difficulties of the subject, he is very 

 much mistaken. The idea of drawing any inferences regarding 

 the constitution of an organic substance from the nature of its 

 products of decomposition with nitric acid, is one worthy only 

 of the infancy of organic chemistry. Conclusions as to the com- 

 position of cane-sugar, for instance, drawn from a consideration 

 of that of oxalic acid, would not be accepted at the present day. 

 Nevertheless the formation of phtalic acid as a product of decom- 

 position of alizarine is sufficient in itself to prove that M . Lau- 

 rent's view of the composition of rubian is perfectly untenable. 

 M, Laurent considers rubian to be a compound of sugar, aliza- 

 rine and rubiretine minus water; just as helicoidine may be 

 regarded as a compound of sugar, hydruret of salicyle, and sali- 

 gerine minus water. Now if this view were correct, on acting 

 on rubian with nitric acid, the sugar would be converted into 

 oxalic acid, the alizarine would yield phtalic acid ; and the rubi- 

 retine, which is not affected by dilute nitric acid, nor in any 

 great degree even by concentrated acid, would be obtained as an 

 insoluble residue. Now on decomposing rubian with nitric acid, 

 the only solid product obtained is phtalic acid. Not a trace of 

 oxalic acid is contained in the liquid, not a trace of matter inso- 

 luble in cold nitric acid is formed. Therefore neither sugar nor 

 rubiretine can be considered as constituents of rubian. The 

 same experiment proves that neither rubianine nor verantine 

 exist ready formed in rubian ; for as neither of them is decom- 

 posed by nitric acid (rubianine being soluble in boiling nitric 

 acid, and crystallizing out again on the acid cooling), they would, 

 if they did, be obtained after the action is completed in the shape 

 of an insoluble residue. 



Pursuing his favourite idea that phtalic acid is the bond which 

 unites all the compounds derived from madder, M. Laurent 

 arrives at the following conclusions. He takes it for granted that 

 alizarine, by the action of nitric acid, is changed into phtalic 



