SOURCES OF AMMONIA. 205 



SUPPLEMENTARY CHAPTERS. 



I. — The Sources of Ammonia. 



When animals appeared on the surface of the earth, it cannot be 

 doubted that means must have been provided for their sustenance 

 and increase, or in other words, that plants must have existed to 

 furnish them with food. But it is quite as obvious that, at the 

 period of the formation of the vegetable world itself, the condi- 

 tions must have existed in the soil and in the atmosphere, neces- 

 sary for the exercise of vegetable life. With the same certainty 

 with which we presuppose the existence of a compound of carbon 

 to furnish that element to vegetation, we must also assume the 

 contemporaneous presence of a compound of nitrogen, such as at 

 the present day yields that element to plants. 



If we disregard the fundamental principle on which all 

 inquiries into nature ought to proceed, then we may assume, 

 a priori, according to our will and pleasure, that other compounds 

 of carbon, differing from carbonic acid, formerly took part in the 

 vital processes of plants ; but if we still retain the foundation of 

 all scientific inquiry, namely, induction from facts, then we can- 

 not admit the existence of these hypothetical compounds of car- 

 bon, either because they are totally unknown to us, or that their 

 existence is doubtful. 



The same reasoning must be adopted in the case of nitrogen. 

 Science is at present ignorant of any compound of nitrogen be- 

 sides ammonia, capable of yielding nitrogen to wild plants on all 

 parts of the earth's surface. No other such compound of nitrogen 

 has been indicated, or even hypothetically supposed to exist, and 

 designated by a name, in the case of cultivated plants ; and there- 

 fore, until a second source of nitrogen is discovered, we must, in 

 sciencer'^iew. ammonia as the only., source., • \ , ... ' , : 



