342 
Book V. 
Properties. 
Crystallizes 
with nitric 
acid. 
Action of 
heat. 
ANIMAL SUBSTANCES. 
very little was known concerning its nature till Four 
croy and Vauquelin published their experiments on it 
in 1799. These celebrated chemists have given it the 
name of urea, which has been generally adopted. 
Urea, obtained in this manner, has the form of cry 
stalline plates crossing each other in different directions. 
Its colour is yellowish-white : it has a fetid smell, some 
what resembling that of garlic or arsenic ; its taste is 
strong and acrid, resembling that of ammoniacal salts ; 
it is very viscid and difficult to cut, and has a good 
deal of resemblance to thick honey*. When exposed 
to the open air, it very soon attracts moisture, and is 
converted into a thick brown liquid. It is extremely 
soluble in water; and during its solution a considerable 
degree of cold is produced†. Alcohol dissolves it with 
facility, but scarcely in so large a proportion as water. 
The alcohol solution yields crystals much more readily 
on evaporation than the solution in water. 
When nitric acid is dropt into a concentrated solution 
of urea in water, a great number of bright pearl-co 
loured crystals are deposited, composed of urea and ni 
tric acid. No other acid produces this singular effect. 
The concentrated solution of urea in water is brown, 
but it becomes yellow when diluted with a large quan 
tity of water. The infusion of nutgalls gives it a yel 
lowish brown colour, but causes no precipitate. Nei 
ther does the infusion of tan produce any precipitatet. 
When heat is applied to urea, it very soon melts. 
swells up and evaporates with an insupportably fetid 
odoúr. When distilled, there comes over first benzoic 
* Fourcroy and Vauquelin, Ann. de Chim. xxxii. 87. 
Ibid. p. 88. 
t Ibid.