203 HlSTOIiY OF 



evaporation, to mix in tliis great alembic, and to float a while in 

 common. Here minerals, from their lowest depths, ascend in 

 noxious or in warm vapours, to make a part of the general 

 mass ; seas, rivers, and subterranean springs, furnish their copi- 



Berzilius, cannot well be accounted for, without supposing hydrogen to be a 

 coustitufnt of it. The specific gravity of azotic gas, according to Kirwaii, is 

 O.StSj, that of air being 1.000; while, according to Lavoisier and Davy, it is 

 9',S. Tills la>t estimate we are disposed to consider as most correct. If so, 

 100 cubic inches of it, at the temperature of 6C» wlien the barometer stands 

 at 30 inches, weigh 29.83 grains troy. Reckoiiinif the specific gravity of 

 oxygen gas 1.093, and that of azotic gas iHS, and Mijjposiiig atmospherical 

 air to be composed of 79 parts of azote and 21 oxy^'on by bulk, it follows, that 

 100 parts of it in weight are composed of 



77.43 azote 



22.57 oxygen 



100.00 

 The third constituent of the atmosphere is carbonic acid gas. Its presence 

 in the atmosphere was rccogni-sed as soon as Dr Black had ascertained the 

 ciiuse of the ditference between raild and caustic alkalies : for it was known, 

 that a caustic alkali soon becomes mild by exposure to the air. Dr Black 

 ascertained, that the mihhie.ss is owing to the absorption of carbonic acid. 

 From the observations of Sanssure « e learn, that this gas exists in the atmo- 

 sphere on the summit of Mount Blanc, which is nearly 1(5,000 feet above the 

 level of the sea; for lime-water soon deposited its lime in the state of caibo- 

 nate, when exposed upon the summit of that mountain, (Saussure's Voyges, 

 iv. 199.) Humboldt found it in a quantity of air bmught down by Garnerin 

 from a height of 4280 feet, to ivhich he had ascended in an air-balloon. {Jour. 

 de Pht/s. xlvii. 202.) It appears, therefore, to constitute a part of every por- 

 tion of the atmosphere to whicli we have access. As this acid gas is pro- 

 duced in great quantities by combustion, respiration, fermentation, and many 

 other of the most common processes of nature, one would be disposed to be. 

 lieve, at first view, that its quantity must be constantly increasing. But this 

 does not appear to be the case ; it must therefore be decomposed and separat- 

 cd from air as fast as it is formed. It is of so deleteriims a nature, that, if it 

 were to accumulate to any extent, it would render air incapable of support, 

 ing life. A candle will not burn in air contaminated with one.ninth of car. 

 bonic acid gits. The quantity of tliis gas in air is small. Many attempts have 

 been made to ascertain it; but the process is so difficult, that absolute pre- 

 cision cannot be looked for. It was long believed that the carbonic acid pre- 

 sent in the atmosphere amounted to one per cent. Humboldt made many 

 experiments on tlie subject, and concluded from them, that its bulk varied 

 from one per cent, tn half a per cent. But this determination is certainly 

 excessive. According to the experiments of Mr Dalton, a quantity of air, 

 equal in bulk to 102,400 grains of water, contains a quantity of carbonic acid 

 just capable of saturating 125 grains of lime-water ; 70 measures of carbonic 

 acid gas would produce the same effect : Hence he concludes, that the atmo. 

 .sphere contains j;^ij^th partof its bulk of carbonic acid gas. {Phil. Mng. xxiii 

 354.5 Tliis quantity we consider as rather below the truth. Mr Cavendish 



