the Colouring Matters of Madder. 191 



acid and oxalic acid ; and as the formula of rubiretine differs 

 from that of alizarine, according to him, merely by one equiva- 

 lent of H, it may easily be conceived that, when exposed to the 

 influence of oxidation, it may yield the same products : — 

 Alizarine. Phtalic acid. Oxalic acid. 



C io H s o 3 + HO + 40 = C 8 H 3 O 4 + C 2 H0 4 . 



Rubiretine. Phtalic acid. Oxalic acid. 



io H 4 3 + 50 = C 8 H 3 O 4 + C 2 HO 4 . 



In like manner purpurine, the existence of which M. Laurent 

 still admits, and rubiacine may, when subjected to the action of 

 nitric acid, resolve themselves into the same ultimate products, 

 since purpurine may be considered as an amide (to use M. Lau- 

 rent's expression) of phtalic acid and alizarine : — 



Purpurine. Phtalic acid. Rubiretine. 



C 18 H 6 O 6 = C 8 H 3 O 4 H _C ]0 H 4 O 3 -HO. 



Rubiacine. Phtalic acid. Alizarine. 



C 18 H 6 O 6 =C 8 H 3 O 4 +C 10 H 3 O 3 -HO. 



Now every one of these conclusions is entirely hypothetical, 

 their only basis being an accidental correspondence between the 

 formulae invented by M. Laurent. In the first place, I may 

 remark that alizarine, by the action of nitric acid, yields phtalic 

 acid only, without a trace of oxalic acid. It is certainly true, 

 that, in acting on madder or garancine with nitric acid, oxalic 

 acid is also obtained as one of the products ; but its formation 

 is due to other constituents of the root, not to the alizarine. In 

 this respect, therefore, the presumed analogy between alizarine 

 and chloronaphtalic acid does not hold good. As regards rubi- 

 retine and rubiacine, anyone reading my description of these 

 bodies with attention, will, I have little doubt, arrive at the con- 

 clusion that their conversion into phtalic acid by means of nitric 

 acid is extremely improbable, if not impossible. Rubiretine is 

 not sensibly affected by boiling dilute nitric acid. Concentrated 

 nitric acid in a boiling state changes it slowly into a yellow sub- 

 stance, which is insoluble in water, and very little soluble in 

 alcohol. Rubiacine dissolves in boiling dilute nitric acid, and 

 crystallizes out again unchanged on the solution cooling. Con- 

 centrated nitric acid decomposes it with difficulty. Moreover, 

 by the action of persalts of iron, rubiacine passes into rubiacic 

 acid, whereas alizarine by the same agency is changed into 

 phtalic acid. As regards the so-called purpurine, I think I 

 have proved synthetically as well as analytically that this sub- 

 stance is a mixture in variable proportions of alizarine and veran- 

 tine, and that therefore its forming phtalic acid when decomposed 

 by nitric acid is not remarkable. I may state, nevertheless, for 

 the information of those who are still inclined to believe in the 



