SCIENCES APPLICABLE TO FARMING. 49 



wind, must soon be exhausted. But here a very wonderful law 

 of nature completely provides for the difficulty. If several 

 sorts of gas or air be brought together, even though some be 

 much Keavier than otlicrs, they will soon become equally mixed 

 tliroughout ; and if any one of them be removed from a partic- 

 ular spot, what remains of the same gas in other portions of the 

 mixture will instantly expand, till it has filled the whole space, 

 just as it would do if that were the only gas present. So that, 

 if all the carbonic acid around a particular plant be absorbed, 

 the gas will rush in from other parts of the atmosphere, and 

 tlius keep a constant supply within reach. In this way a tree 

 can go on without interruption, except by winter, accumulating 

 carbon for years, and even centuries. A single pine tree in 

 Oregon, for instance, is sometimes found to contain 256,000 

 pounds of carbon ; which required 1,305,333 pounds of carbonic 

 acid ; two-thirds of which is 870,222 pounds, all taken from the 

 atmosphere ; or 800 pounds yearly, on the supposition that the 

 tree required 1,100 years for its growth. This single example 

 will give some idea of the magnitude of the process that is going 

 on, silently, yet surely, to supply all the forests on the globe. 



So much for the carbon which forms the principal part of the 

 solid portion of the plant. Whence does it obtain its oxygen 

 and liydrogen ? Nearly all of it, no doubt, from the water 

 pumped up by the roots, or absorbed by the leaves ; for water is 

 entirely composed of these two elements. Nitrogen, also, the 

 least abundant ingredient, might, it would seem, be derived 

 directly from the air by absorption, since four-fifths of the 

 atmosphere consist of it ; but there is no evidence of any such 

 absor|)tion. Yet a small quantity of it is absorbed by the water 

 taken up by the roots. Ammonia, also, a compound of nitrogen 

 and hydrogen, sometimes exists in small quantity in the air, and 

 is produced still more abundantly by fermenting manures. 

 Nitric acid, likewise, is sometimes found in minute quantity in 

 the atmosphere, and its absorption would furnish nitrogen as 

 well as oxygen. 



As to the inorganic matter of plants, the sulphur, phosphorus, 

 lime, silcx, iron, manganese, (fee, it must nearly all be derived 

 from the soil, since water alone can hold it in solution. Chlo- 

 rine perhaps may, as Dr. Dana suggests, be derived from the 

 7 



