riii: EAKTH. 207 



CHAP. XIX. 



AN ESSAY TOWARDS A NATURAL HISTORY OF THE Ala. 



A LATE eminent philosopher has considered our atmosphere as 

 one large chymical vessel, in which an infinite number of various 

 opeiations are constantly performing. In it all the bodies of 

 the earth are contitmally sending up a part of their substance by 



concliisioos of Scheele and Lavoisier ; and it was not till towards the com. 

 menceraent of tlie I9th century, that the true proportion of these constituents 

 was generally known. The experiments of BerthoUet in Egypt and in Paris, 

 seem to have led the way to it. These were almost immediately contirroed 

 hy *hose of Davy, Beddoes, and many other chemists. At present It is uni. 

 vorsally admitted, that atmospheric air never varies in its composition; that 

 it Is the same in all places, and in all seasons ; and that it consists in bulk of 



79 azote 



21 oxygen 



100 

 priiportions almost exactly the same with those originally settled by Mr 

 Cavendish. 



Oxygen gas is undoubtedly the most important of the constituents of the 

 atmosphere, and indeed one of the most remarkable substances in nature, and 

 highly worthy of the investigation of the chemist. Dr Priestley, its original 

 discoverer, gave it the name of dephlogisticated air, Scheele called it empy. 

 •real air, Lavoisier called it at first highly respirable air, then vital air, and at 

 last oxygen gas, because he considered it as the acidifying principle. It pos.. 

 sesses the mechanical properties of common air ; combustibles burn in it with 

 great brilliancy ; and animals can breathe it much longer than the same quan- 

 tity of common air. If the specific gravity of common air be reckoned 1.000, 

 that of oxygen gas, according to the experiments of Kirwin and Lavoisier, is 

 1.103; according to Davy, 1.127; according to Fourcroy, Vauquelin, and 

 .Scguin, 1.0S7 ; and according to Allen and Pepys, 1.090. The.se results do 

 not differ much from each other, except that of Mr Davy. His oxygen was 

 obtained from the black oxide of manganese, and might perhaps contain a little 

 carbonic acid pas. If we exclude his. the average of the other three is 1.093. 

 'I'his may be considered as near the truth as can well be attained. Rating its 

 specific gravity at 1.093, 100 cubic inchesof it, at the temperature of 60" wheJi 

 the barometer stands at 30 inches, will weigl) 33 j grains troy. 



Azotic gas, the other constituent of atmospherical air, is chiefly recognised 

 by its negative qualities. It possesses the mechanical properties of air ; it 

 does not support combustion ; and no animal can breathe it without death. 

 It constitutes the base of nitric acid, and is one of the constituents of ammonia. 

 There is reason to consider it as a compound body, but hitherto chemists have 

 not lieen able to ascertain its constituents ; though several extraordinary 

 phenomena, observed during the decomposition of ammouia by Davy aud 



s2 



