FIRST REPLY. 135 



been discovered, that even to its thousandth parts, the air, 

 wheresoever it has been examined, has always the same 

 composition. 



But conclusions relating to the vital processes of plants 

 and animals have been very hastily deduced from this 

 constant composition of the atmosphere. Our atmosphere, 

 according to Poggendorf s estimate, contains about 1,9 5 4,5 7 8 

 cubic geographical miles of oxygen, while the respiration 

 of man and animals, together with the various processes 

 of combustion, consume annually about 24 cubic miles ; 

 consequently 250 cubic miles in a hundred years, or only 

 nearly a ten-thousandth part. Our instruments, however, 

 would not mark a diminution of so small an extent, even 

 were they ever so accurately constructed, and carefully used 

 for centuries. Our methods of determining the amount of 

 carbonic acid in the air admit of far greater accuracy, and 

 a much more certain estimate has thus been obtained, 

 applicable, as will subsequently be seen, to the same deduc- 

 tions. In respiration, for every cubic inch of oxygen he 

 inspires, a man expires a cubic inch of carbonic acid, and 

 exactly the same exchange occurs in processes of combus- 

 tion. According to this assumption, about 12,500 cubic 

 geographical miles of carbonic acid have been breathed out 

 into the air in the course of five thousand years, leaving 

 out of the question the vast quantity which yearly streams 

 forth out of volcanoes. The carbonic acid in the air, there- 

 fore, should be in proportion to oxygen as 1 to 155, while 

 in reality, it amounts to but one-fourth per cent. It is 

 clear from this, that some process must exist by which the 

 carbonic acid is extracted from the air, and brought into 

 some other combination. 



Oxygen has the property of combining readily with 

 other substances, especially with carbon and hydrogen; a 

 process which chemists call combustion, even though it is 



