INDIGO-BLUE. 383 



tals, difficultly soluble in cold water, more easily solu- 

 ble in hot water ; fuses at 110 ; combines with acids 

 by heating, water being eliminated and bodies, like 

 ethers, being formed. 



Piperic acid, C 12 H 10 4 . The potassium salt is 

 formed together with piperidin by boiling piperin (see 

 Alkaloids) with alcoholic potassa. Hydrochloric acid 

 throws down the free acid from the solution of the 

 salt. Crystallizes from alcohol in long, capillary 

 needles ; almost insoluble in water, but slightly soluble 

 in cold alcohol, more easily in hot alcohol ; fuses at 

 216-217 and sublimes, at the same time undergoing 

 partial decomposition. Monobasic acid ; yields proto- 

 catechuic (p. 356), acetic, and oxalic acids when fused 

 with potassium hydroxide. Potassium bichromate and 

 dilute sulphuric acid destroy it completely with the 

 aid of gentle heat, forming carbonic anhydride and 

 water. Potassium hypermanganate converts it into 

 piperonal (p. 324). 



Hydropiperic acid, C 12 H 12 O. Is formed from 

 piperic acid by direct combination with nascent hydro- 

 gen. Long, colorless, fine needles, fusing at 70-71. 



FOURTH GROUP (INDIGO-GROUP). 



Indigo-blue, C 8 H 5 NO or C 16 H 10 K 2 2 . Indigo is ob- 

 tained from the various species of Indigqfera of East 

 India and South America, from Isatis tinctoria, Nerium 

 tinctorium, Polygonum tinctorium, and other plants. The 

 bk>ssoming plants are cut oft' and allowed to 'stand 

 under water from twelve to fifteen hours. The liquid 

 is then drawn oft', and, by means of beating with 

 wooden shovels, etc., brought in contact with the air 

 as much as possible. The indigo, which separates dur- 

 ing this process, is separated from the brown liquid, 

 boiled with water, and dried. Indigo is not contained, 

 ready formed, in the plants. From what compound 



