ATMOSPHERIC <\IR. 47 



to be no longer capable of supporting combustion. 

 The reason of this is, that thtj oxygenous part of 

 the air has united to the metal, and converted it 

 into an oxyde, leaving behind only the nitrogen. 



This decomposition of the atmospheric air may 

 be effected more easily by burning phosphorus in 

 it. During the combustion of the phosphorus, it 

 unites to the oxygen, and forms phosphoric acid ; 

 the remainder is nitrogen. 



The proportion of oxygen gas contained in a 

 given quantity of atmospheric air can be ascertain- 

 ed by various processes. One method is, by in- 

 verting a glass tube into a solution of sulphuret of 

 potass. This substance will absorb the oxygen 

 gas, but not the nitrogen ; hence the air in the 

 tube will diminish in bulk, and what remains will 

 show the proportion of nitrogen. 



It was supposed by modern chemists, until lately, 

 that oxygen was essential to combustion ; and that 

 this process was, in all cases, the combination: of 

 oxygen with the combustible body; but it ihas 

 been found, that there are some other substances, 

 as chlorine and iodine, which have also this pro- 

 perty of supporting combustion : it is, however, t he 

 oxygen that acts in all the usual combustions in 

 common air. The heat and light were supposed 

 to be separated from oxygen, the base of the ga.s, 

 which became fixed in the burned body. At pre- 

 sent it is maintained by some, that combustion 

 may be the consequence of intense chemical ac- 

 tion, and need not depend upon any particular 

 combination. This subject, however, remains very 

 obscure. 



Air, which has been breathed, is found to have 

 lost its oxygen. This principle is retained in the 



