CHEMICAL CONSTITUTION OF THE ATMOSPHERE. 323 



of Devonshire, or in the air of the city of London. This 

 constancy in constitution leads to the* supposition that 

 the oxygen and nitrogen are chemically combined ; but 

 many eminent philosophers have contended that they 

 are merely mechanically mixed ; arid they have shown 

 that some peculiar properties prevail amongst gaseous 

 bodies, which very fully explain the equal admixture of 

 two gases the specific gravities of which are different. 

 This is particularly exemplified in the case of carbonic 

 acid, of which gas one per cent, can be detected in all 

 regions of the air to which the investigations of man 

 have reached. This gas, although so heavy, is, by the 

 law of diffusion, mixed with great uniformity throughout 

 the mass.* Every exhalation from the earth, of course, 

 passes into the air ; but these are generally either so 

 light that they are carried into the upper regions, and 

 there perform their parts in the meteorological pheno- 

 mena, or they are otherwise very readily absorbed by 

 water or growing plants, and thus is the atmosphere 

 preserved in a state of purity for the uses of animals. 

 Again, the quantity of oxygen contained in the air, and 

 its very peculiar character, ensures the oxidation of all 

 the volatile organic matters which are constantly passing 

 off, as the odoriferous principles of plants, the mias T 

 mata of swamps, and the products of animal putrefac- 

 tion ; these are rapidly converted into water, carbonic 

 acid, or nitric acid, and quickly enter into new and harm- 

 less combinations. The elements of contagion we are 

 unacquainted with ; but since the attention of inquirers 

 has been of late directed to this important and delicate 

 subject, some light may possibly be thrown upon it 

 before long. 

 Nothing shows more strikingly the admirable adaptation 



* On the Law of Diffusion of Gases : by Thomas Graham, M.A., 

 F.R.S., Ac. ; Edinburgh Philosophical Transactions, 1832. Sur 

 I Action Capillaire des Fissures, fyc. : by Dobereiner; Annales de 

 Chimie, xxiv. 332. 



