52 



Atmo. 

 sphere 



Bulk of 

 oxygen 



and azo- 

 tic gas. 



Properties 

 >f oxygen. 



knowledge in his rohime of essays published after 

 that period, and as he was entirely unacquainted with 

 oxygen gas when Priestley shewed him the way to 



prepare it at Paris, about the end of 1771. It is 

 very probable that Lavoisier became acquainted with 

 imposition of atmospherical air not very long 

 after that period ; though some years elapsed before 

 he made it known to the public. Whether he pre- 

 ceded Scheele in his knowledge of this important 

 fact, we do not exactly know. But there is no doubt 

 whatever, that Scheele's investigations were carried 

 on without any assistance from abroad, and that it 

 was in consequence of the publication of his Trea- 

 tise on Air nui! Fire, that the chemical world became 

 acquainted with the nature ar.d composition of at- 

 mospherical air. This important work was printed 

 at Upsal in 1777, with an introduction by Bergmann, 

 and translated into English by Dr Foster in 1780. 

 The experiments of Priestley indeed would have war- 

 ranted the conclusions respecting the composition of 

 atmospherical air drawn by Scheele ; but those of 

 Dr Priestley were different and more complicated. 

 In Scheele's first experiments, he extirpated the bulk 

 of oxygen gas in air at 30 per cent. But in the year 

 1779, he published a set of experiments continued for 

 a whole year, in order to ascertain whether the bulk 

 of oxyen in air be constant, or varies with the season 

 of the year. He found it in general remarkably con- 

 . stant, and amounting to 27 per cent. The smallest 

 bulk was 2-1, and the greatest observed was 30 per 

 cent. Dr Priestley had made similar experiments, 

 and had estimated the bulk of the oxygen at fth of 

 the air, or 20 per cent. Mr Lavoisier's experiments, 

 which were very numerous and varied, almost coin- 

 cided with those of Scheele. He considered air as 

 composed of 27 parts by bulk of oxygen, and 73 of 

 azote. Mr Cavendish's experiments were published 

 in the Philosophical Transactions for 1783. He 

 proved decisively, that the proportion of the azote 

 and oxygen in the atmosphere does not vary ; and by 

 a very careful analysis, concluded, that 100 parts of 

 air in bulk are composed of 

 79.16 azote 

 , 20.81 oxygen 



100.00 

 This opinion was not at first acceded to by chemists, 

 misled by the previous conclusions of Scheele and 

 Lavoisier ; and it was not till towards the commence- 

 ment of the 19th century, that the true proportion 

 of these constituents was generally known. The 

 experiments of Berthollet in Egypt and in Parts, 

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

 immediately confirmed by those of Davy, Beddoes, 

 and many other chemists. At present it is univer- 

 sally admitttd, 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 

 proportions almost exactly the same with those ori- 

 ginally .settled by Mr Cavendish. 



Oxygen gas is undoubtedly the most important 

 of the constituents of the atmosphere, and indeed one 



ATMOSPHERE. 



of the most remarkable substances in nature, and Atmo- 

 highly worthy of the investigation of the chemist. 'I' i " rc ' 

 Dr Priestley, its original discoverer, gave it the name * 



of dephlngi ticutrtl nir, Scheele called it empyreal air, 

 Lavoisier called it at first highly rexpnabtc air, then 

 vital air, and at last oxygen gk*i because he consi- 

 dered it as the acidifying principle. It posses 

 the mechanical properties of common air; combus- 

 tibles burn in it with great brilliancy ; and animals 

 can breath it much longer than the same quantity of 

 common air. If the specific gravity of common air 

 be reckoned 1.000, that of oxygen gas, according 

 to the experiments of Kirwan aid Lavoisier, is 

 1.103 ; according to Davy, 1.127 ; according to 

 Fourcroy, Vauquelin, and Seguin, 1.067; and ac- 

 cording to Allen and Pepys, 1.090. These results 

 do not differ much from each other, except that of 

 Mr Davy. His oxygen was obtained trom the black 

 oxide of manganese, and might perhaps contain a 

 little carbonic acid gas. If we exclude his, the ave- 

 of the other three is 1.093. This may b?> con- 

 sidered as near the truth as can well be attained. 

 Rating its specific gravity at 1.093, 100 cubic inches 

 of it, at the temperature of t>0 when the barometer 

 stands at 30 inches, will weigh 'M'\ grains troy. 



Azotic gas, the other constituent of atmosphe- Properties 

 rical air, is chiefly ivco gn is-.d bv its negative qualities, of azote. 

 It possesses the mechanical properties of air ; it does 

 not support combustion ; and no animal can breath 

 it without death. It constitutes the base of nitric 

 acid, and is one of ^he constituents of ammonia. 

 There is reason to consider it as a compound body, 

 but hitherto chemists have not been able to ascertain 

 its constituents ; though several extraordinary phe- 

 nomena, observed during the decomposition of am- 

 monia by Davy and Berzelius, cannot well be ac- 

 counted for, without supposing hydrogen to be a con- 

 stituent of it. It has been supposed a compound of 

 hydrogen and oxygen ; but several circumstances 

 militate against this opinion. Mr Davy has been for 

 some time occupied incessantly in attempts to ascertain 

 its composition, but hitherto without success. Till the 

 discovery be made, some of the most interesting parts 

 of chemistry remain involved in impenetrable obscurity. 

 The specific gravity' of azotic gas, according to Kirwan, 

 is 0.985, that of air being 1.O00; while, according' 

 to Lavoisier and Davy, it is 0.978. This last esti- 

 mate we are disposed to consider as most correct. If 

 so, 100 cubic inches of it, at the temperature of r>0 

 when the barometer stands at 30 inches, weigh 29.83 

 grains troy. 



Reckoning the specific gravity of oxygen gas Relative 

 1.093, and that of azotic g- weight of 



atmospherical air to b ed of 79 parts of azote ,J ! C con_ 



and 21 oxygen by bulk, it follows, that 100 parts of slltueut5, 

 it in weight are composed of 



77.13 azote 

 82.57 oxygen 



L00 



Though it has been ascertained, that these two 

 constituents of air n v.r vary in their proportions, yet F.udiome- 

 as the methods of analysing air are very useful in all ,er - 

 chemical invest if gaseous bodies, ar.d have 



led to many di i f importance, it will be pro- 



per to give an account of them here. They consist 



