INFLUENCE OF SOILS. 125 



more permeable to water ; whilst, on the other hand, a loose 

 sandy soil may be advantageously tempered with clay. 



179. The supply of moisture to the roots, however, is not the 

 only important object, which the soil should answer. It ought 

 to afford carbonic acid also ; since it is essential to the rapid 

 growth of a Plant, that this part of its nourishment should be 

 taken in by its roots, as well as by its leaves. The carbonic 

 acid may be furnished in two ways ; either the soil may absorb 

 it from the atmosphere ; or the decay of some of the matter 

 contained in it, may disengage this product. There is a remark- 

 able property possessed by several porous substances, of absorb- 

 ing gases, and especially carbonic acid gas, to the amount of 

 many times their own bulk. (See MECHAN. PHILOS., . 33.) 

 Of all these, charcoal is one of the most powerful in this respect ; 

 and it has been found, that many plants may be grown in pow- 

 dered charcoal, if sufficiently supplied with water, more luxu- 

 riantly than in any other soil. The charcoal itself undergoes no 

 change, but it absorbs carbonic acid gas from the air : this is 

 dissolved by the water which is taken up by the roots, and thus 

 it is introduced into the system. In such cases, the plant 

 derives its solid matter as completely from the atmosphere alone, 

 as if its roots were entirely exposed to it ; for not a particle of 

 the charcoal is dissolved, and it, therefore, affords no nutriment 

 to the plants. 



180. It may be thought incredible that the enormous quan- 

 tity of carbon, which enters into the composition of a single tree, 

 - much more of an extensive forest, and much more still of the 

 immense succession of such luxuriant forests, as those which 

 formed our beds of coal, should ever have been contained in 

 the atmosphere ; since any given quantity of air contains only 

 about one-thousandth of its weight of carbonic acid ; and this 

 gas is composed of only about 27 parts of solid carbon in every 

 100. But it must be remembered that, as the weight of the air 

 pressing upon every square inch of the earth's surface is 151bs., 

 that pressing upon a square foot will be 21601bs. ; and as the 

 surface of the earth can be almost exactly calculated, it may be 

 shown that, in the whole of the atmosphere surrounding it, at 



