48 SUPPLIES OF ORGANIC FOOD. 



It will now be seen what an important part these 

 four elements act, in the economy of nature. From 

 them all the forms of vegetable life are built up; they 

 are constantly passing from one state of combination 

 into another, and yet always come out at last them- 

 selves unchanged. This is for the reason that they 

 are truly, and not in the common sense, elementary 

 bodies. If we take a piece of wood for examination, 

 Ave can divide it by various means into oxygen, carbon 

 and hydrogen; but we fail in any attempt to subdivide 

 again either of these three bodies. Those bodies then 

 are elementary, chemically speaking, which w r e can 

 not by any means decompose or separate, which we 

 can not show to be compound. There are in all be- 

 tween fifty and sixty of these elements known, and 

 among them are the four gases the functions of which 

 we have been considering. Sulphur and phosphorus 

 are also elements. 



SECTION VI. OF THE SUPPLIES OF ORGANIC FOOD TO 

 PLANTS. 



The sources from whence plants derive their various 

 kinds of organic food, are different in different locali- 

 ties. 



Carbon is mostly drawn in from the air in the form 

 of carbonic acid : some also comes from the soil, but 

 by far the greater part from the air. The quantity 

 required for the support of all the vegetation upon the 

 earth's surface must be immense, especially when w r e 

 know T the fact that carbon in general constitutes fully 

 half, and sometimes much more than half of its weight. 

 When w r e remember that the proportion of carbonic 

 acid in the air is but about j^^h of all, there may 

 seem to be some danger of its exhaustion. 



It has been said that the weight of this gas in the 

 air over every acre of the earth's surface, is about 

 seven tons. This quantity, if the land were all under 



