ADDRESS. 11 



in plants, is absorbed directly from the atmosphere by their 

 leaves. And yet only l-'SSOOth part of the atmosphere is car- 

 bonic acid, which is diffused through the whole air, and less 

 than one third of this gas is carbon. How then can a tree or 

 forest, for instance, obtain enough of this gas to form so large 

 a part of its substance ? since the quantity immediately around 

 it, and even the fresh portions brought by the wind, must soon 

 be exhausted. But here a very wonderful law of nature com- 

 pletely provides for the difficulty. If several sorts of gas or 

 air be brought together, even though some be much heavier 

 than others, they will soon become equally mixed throughout ; 

 and if any one of them be removed from a particular 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 thus 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 at- 

 mosphere ; or 800 pounds yearly ; on the supposition that the 

 tree required 1100 years for its growth. This single example 

 will give some idea of the magnitude of the process that is go- 

 ing 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 oxy- 

 gen and hydrogen ? 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 consists sf it ; but there is no evidence of any 

 such absorption. Yet a small quantity of it is absorbed by the 

 water taken up by the roots, Ammonig also, a compound of 



