Jan. 1, 1865.] THE TECHNOLOGIST. 



ON CHEMISTRY APPLIED TO THE ARTS. 285 



principal source of gravel and calculus, owing to various influences 

 which make the urine strongly acid before its rejection, whereby the 

 soda is neutralised, the uric acid liberated, and this being nearly 

 insoluble separates, and has a tendency to form gravel or calculus. In 

 fact, the deposit which occurs in this fluid is generally represented by 

 uric acid, phosphate of lime, and magnesia, mucus, and colouring 

 matter. It may be here stated that calculi were formerly held in great 

 estimation, especially those formed in the intestine and called bezoars, 

 and this was the case in Eastern countries until very recently. Thus it 

 is related that a Shah of Persia sent to Napoleon the First, among other 

 valuable presents, three bezoars, which were considered to be of great 

 antiquity, and capable of curing all diseases. The urine of birds and 

 reptiles being almost entirely composed of urate of lime explains why 

 their refuse is of such value as a manure, which arises from its trans- 

 formation into carbonate of ammonia. When large masses of this 

 refuse undergo a slow and gradual decomposition, as in the dry climate 

 of the Pacific Islands, on the coasts of Peru and Chili, it constitutes 

 guano. It may be interesting to know that in 1835, 6, and 7, a most 

 beautiful colour was prepared from the uric acid contained in guano, 

 and used largely by calico-printers and silk-dyers under the name of 

 Eoman purple, or murexide. 



Before leaving the study of this important animal secretion, let me 

 say a few words on the urine of herbivorous animals. It is generally 

 alkaline, and contains, besides an aromatic principle, an acid discovered 

 by Liebig, and called hippuric acid, together with urea and uric acid, 

 also found in human urine. Hippuric acid is easily obtained in the 

 form of well-defined crystals, by rapidly evaporating the fluid con- 

 taining it. This acid does not exist in the food of the animal ; but 

 benzoic acid, or its homologues, are found there, and during the pheno- 

 mena of digestion the nitrogenated principles produced by the wear and 

 tear of life fix themselves on the benzoic acid, and convert it into hip- 

 puric acid, as seen by this formula : — 



Benzoic Acid. Hippuric Acid. 



C 14 H 5 3 + H0 ... C 18 H 8 5 N 4 + HO. 



A further proof of the correctness of this view is that when hippuric 

 acid is treated with strong acids or alkali, it transforms itself into 

 benzoic acid, which can be easily extracted. 



