RETENTION OF HEAT. 73 



of the organic matter of the soil, which is often 

 observed during the growth of plants, — but also by 

 the general view of the earth's surface just taken ; 

 because there is nothing in its aspect to warrant 

 the idea that its means of maintaining the organic 

 kingdoms are declining with the rapidity indicated 

 in the statements just made.i 



Although there is no doubt that plants depend 

 largely upon the atmosphere as well as on the soil for 

 their carbonic acid, still there is probably no fixed 

 law which governs the exact proportion which it 

 shall receive from either, under all circumstances. 

 If the soil is wanting in this element, or in the 

 power of setting it free by decomposition, there is 

 reason to believe that the foliage will endeavor to 

 make up for the deficiency by imbibing it from the 

 atmosphere. If, on the other hand, the soil is over- 

 charged with carbonic acid, it may be also true that 

 the leaves of those plants which are seen upon its 

 surface exude, like a safety-valve, the surplus ma- 

 terial. Such a theory, it appears to us, is consistent 

 with the belief that God made all his natural laws 

 so perfectly adapted to each other that, to all ap- 

 pearances, they are, like the stones which form the 

 arch, self-supporting. 



Absorption and retention of heat. The fact is well 

 known that the earth, by absorbing the rays of heat 



1 Dr. Lindley's quotation from Annals of Natural History. 



7 



