MANURE. 169 



animal acid. This is it which renders a detailed ac- 

 count of these manures interesting to the farmer. It 

 is not enough for his purpose to refer the action of 

 these liquids to the general effect of salts or mineral 

 manures. 



240. The peculiar animal acid to which reference 

 has been made, becomes like nitric acid in nitrates, 

 the food of plants. The element from which it is 

 derived gives a marked and highly valuable charac- 

 ter to the liquid evacuations of the farm yard, and 

 household. This peculiar animal principle is urea. 

 It may be obtained from these liquids, in transparent, 

 but colorless crystals of a faint but peculiar odour. 

 Cold water dissolves more than its weight, and boil- 

 ing water an indefinite quantity of crystals of urea. 

 The pure crystals undergo no change, when dissol- 

 ved in pure water, but if they are mixed with the 

 other ingredients of the urine, decomposition rapidly 

 ensues, and they are resolved almost entirely into 

 carbonate of ammonia. Alkalies and alkaline earths 

 induce similar changes on urea. The practical value 

 of this fact will be easily understood. 



241. Pure urea has no distinct alkaline properties. 

 It unites with aqua fortis, or nitric acid, and forms a 

 sparingly soluble salt, composed of about equal parts 

 of each of its ingredients. 



15 



