6B PHILOSOPHY 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- 
ime laminee, crossing each other in different directions. 
According to Dr Prout, it may be obtained in a state of 
still greater purity by the followimg process: ‘* To the ex- 
tract of trme evaporated to the consistence of a syrup, Di- 
tric acid 1s 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 mtrate of potash or soda may 
crystallise, and be thus separated. The fluid which is left 
is made into a paste with ammal 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 erys- 
tallisation from the alcohol, two or three times *.” 
When prepared according to this last methed, its erys- 
tals assume the form of a four-sided prism, transparent and 
colourless, with a slightly pearly lustre, sp. gr. 1.85. It 
has a peculiar but not urmous odour, and undergoes no 

* Annals of Phil. xi. p. 352. 
