206 HISTORY or 



gold. We might also prove it millions of times purer than upon 

 earth, when raised to the surface of the atmosphere. But thes(« 

 speculations do not belong to natural history ; and they have 

 hitherto produced no great advantages in that branch of science 

 to which they more properly appertain.* 



* Atmospherical air was Ions? considered as a simple elementary body. 

 But it is now known to consist of at least four distinct substances, namely, 

 oxygen, azote, carbonic acid, and aqueous vapour. The first two substances 

 must be considered as its essential constituents, and constitute in fact almost 

 the whole of it The other two are variable in their proportion, and exist only 

 in minute quantities, which it is difficult to appreciate. The first knowledge 

 of the composition of the atmosphere must have been after the period of the 

 discovery of oxygen gas by Dr Priestley, in 1774. Lavoisier, indeed, in 

 his posthumous works, appears to insinuate a knowledge of it in 1772. But 

 this claim caimot be admitted, as he gives no hint of any such knowledge in 

 his volume of essays published after that period, and :\s lie was entirely unac. 

 quainted with oxygen gas when Priestley showed him the way to prepai'e it 

 at Paris, about the end of 1771. It is very probable that Lavoisier became 

 acquainted with the composition of atmospherical air not very long after that 

 period ; though some years elapsed before he made it known to the public 

 Whether he preceded Scheele in his knowledge of this important fact, we do 

 not exactly know. But there is no doubt whatever, that Scheole's investi- 

 gations were carried on without any assistance from abroad, and that it wa.s 

 in consequence of the publication of his Treatise on Air and Fire, that the 

 chemical world became acquainted with the nature and 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 1789. 

 The experiments of Priestley, indeed, would have warranted 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 ex. 

 periments, he estimated 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 oxygen in air be constant, or varies 

 with the season of the year. He found it in general remarkably constant, 

 and amounting to 27 per cent. Tlie smallest bulk was 24, and the greatest 

 observed was 30 per cent. Dr Priestley had made similar experiments, and 

 bad estimated the bulk of the oxygen at ^th of the air, or 20 per cent. Sit 

 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 pub. 

 lished in the Philosophical Traij^actions for 1783. He proved decisively, that 

 the proportion of the azote and oxygen in the atmosphere does not vary ; atid 

 by a very careful analysis, concluded that 100 parts of air in bulk are com. 

 posed of 



79.10 azote 

 20.84 oxygen 



inn.no 

 This opinion was not at first nccfded to by chemists, misled by the previoun 



