INDIGO AND SALICIC COMPOUNDS. 121 



by chlorination, and, after suitable treatment, of striking a purple 

 colour with ferric salts, as you perceive. This last reaction 

 constitutes Piria's well-known test for tyrosine. Now, according 

 to Schmitt and Nasse, the particular member of the salicic sub- 

 group which enters into the constitution of tyrosine is salicic 

 acid, whose formula, you observe, differs from that of benzoic 

 acid by an excess of one atom of oxygen. Moreover, benzoic 

 acid is isomeric with salicic aldehyd, and salicic acid with ampelic 

 or oxi-benzoic acid, thus : 



C 7 H 6 3 Benzoic acid, and salicic aldehyd 

 C 7 H,50 3 Ampelic acid, and salicic acid 



(130.) With regard to the natural history of salicic compounds, 

 salicin is a glucoside of salicylic alcohol, and oil of spiraea con- 

 stitutes salicic aldehyd, while oil of wintergreen is composed 

 largely of methylic salicate, as I have already explained. That 

 some salicic grouping, moreover, occurs as a constituent residue of 

 indigo CgHyTO, is shown by the following considerations. 

 When, for example, indigo experiences decomposition by treatment 

 with reagents, its single atom of nitrogen and one of its eight atoms 

 of carbon are more particularly affected ; and hence, as a convenient 

 representation of its probable molecular constitution, we may as- 

 sociate this mobile carbon and nitrogen with one another, and so 

 write the formula of indigo upon the y-carbon or salicic type, 

 thus : 



C 7 H 5 (CN)0. Indigo or cyan-salicol 



Now, by boiling indigo for a long time with oxidising. agents, 

 and by treating salicic acid with strong nitric acid, we obtain 

 identically the same product, which has received the names of 

 anilic, indigotic, and nitro-salicic acid, thus : 



Indigo Oxygen . Anilic acid Carb-anhyd. 



C 7 H 5 (CN)0 + 6 C 7 H 5 (NO a )0 3 + C0 2 



Salicic acid Nitric acid Nitro-salicic acid Water 



C 7 H 6 3 + (NO^HO = C 7 H 5 (NO a )0 3 + H.HO 



