260 PHYSIOLOGY AT THE FARM. 



same proportions with cyanate of ammonia. When pure, it 

 shows little tendency to decomposition, but brought into 

 contact with substances that act as ferments, it assumes the 

 oxygen and hydrogen of two equivalents of water, and be- 

 comes resolved into carbonate of ammonia ; and as all urine, 

 after it is discharged from the body, undergoes this change, 

 owing to the mucus of the bladder acting as a ferment, the 

 carbonic acid and ammonia which have been decomposed in 

 the production of the organic compounds of plants are 

 restored to the inorganic world.* The urine of man contains 

 also in small proportion uric acid, called also lithic acid, but 

 which has no further connection with urea than that it exists 

 along with it in the secretion of the kidneys. Uric acid is 

 hardly soluble in cold water, but is more soluble in warm 

 water holding phosphate of soda in solution, which is pro- 

 bably its state in natural urine. When combined with am- 

 monia, uric acid is more soluble. In the semi-solid urine of 

 serpents, uric acid is partly conjoined with ammonia, while a 

 portion of it exists, as in the urine of birds, undissolved. The 

 guano found in the islands of the Pacific Ocean, where no rain 

 falls, consists chiefly of urate of ammonia, formed of the excre- 

 ments of birds (urine and faeces) accumulated through count- 

 less generations, and more or less extensively decomposed. 

 When purified, uric acid is a glistening snow-white powder, 

 to appearance amorphous, but when subjected to the micro- 

 scope it exhibits very minute regular crystals. It has neither 

 taste nor smell, and its acid reaction is very feeble. 



In the urine of herbivorous animals uric acid is replaced by 

 hippuric acid, which is also present in the human urine in 

 very small quantities, but more easily detected after using 

 vegetable food, and particularly after taking benzoic acid into 

 the system. Hippuric acid is most readily obtained from the 

 * Carpenter, ' Comparative Physiology, ' p. 435. 



