226 THE CHEMISTRY OF CREATION. 



facturing processes, are also abundant sources of 

 this ingredient. In London it is said to be often 

 seen in the form of an ammoniacal salt, in little 

 star-like spots upon dirty windows, which serves 

 to show that it exists in greater measure in the 

 air of populous cities than elsewhere. 



The principal means by which atmospheric 

 ammonia is rendered available .for the purposes 

 of vegetation, is by its being dissolved in rain- 

 water. It is curious, however, to notice that 

 some manures, which the agriculturist's expe- 

 rience has taught him the value of without 

 revealing its cause, owe some part of their 

 efficacy to the peculiar property* they possess 

 of absorbing ammonia from the air. Gypsum, 

 or sulphate of lime, is a valuable manure, yet it 

 is very insoluble in water; therefore, merely as 

 sulphate of lime, it is not possible that it can 

 directly contribute, to any large amount, to the 

 fertility of our fields. Burnt clay, and pure 

 vegetable mould, are also considered to be of 

 little value as direct fertilizers. It is therefore 

 thought by many chemists, that their principal 

 function is to withdraw ammonia from the air 

 and supply it to vegetables. This is effected in 

 a remarkable manner, which may be illustrated 

 by an experiment. If a piece of freshly-burnt 

 charcoal is put, after cooling, into a glass vessel 

 * Professor Liebig. 



