VHILOSOPHY OF ZOOLOGY. 



the accuracy of the analysis of those substances, of which 

 it is considered as forming an essential ingredient. 



6. Urea. This substance is usually procured from hu- 

 man urine by the following process : Evaporate the urine to 

 the consistence of a thick syrup, which concretes into a crys- 

 talline mass as it cools. Let this be treated with four times 

 its weight of alcohol, at different times, assisted by a gentle 

 heat, and the greater part will be dissolved. By evaporat- 

 ing this solution to a syrupy consistence, the urea remains, 

 and, as it cools, appears in the form of yellowish-white shin- 

 ing laminae, crossing each other in different directions. 

 According to Dr PROUT, it may be obtained in a state of 

 still greater purity by the following process : " To the ex- 

 tract of urine evaporated to the consistence of a syrup, ni- 

 tric acid is to be gradually added until the whole is con- 

 verted to a crystalline mass, which is to be slightly washed 

 with cold water. The nitric acid is then neutralized by a 

 solution of subcarbonate of potash or soda, and the solution 

 evaporated, in order that the nitrate of potash or soda may 

 crystallise, and be thus separated. The fluid which is left 

 is made into a paste with animal charcoal ; cold water se- 

 parates the urea from this paste in a colourless state ; it 

 is to be evaporated to dryness, and the mass digested in 

 alcohol, which takes up the urea alone, leaving the saline 

 bodies or other extraneous matters ; and from the alcoholic 

 solution, the urea may be obtained pure and in the crystal- 

 lised state, although it is often necessary to repeat the crys- 

 tallisation from tlu' alcohol, two or three times V 



When prepared according to this last method, its crys- 

 tals assume the form of a four-sided prism, transparent and 

 colourless, with a slightly pearly lustre, sp. gr. 1.35. It 

 h:is a peculiar but not urinous odour, and undergoes no 



* Annals, of Phil. zi. p. 35?. 



