134 WHAT DOES MAN LIVE UPON? 



and oxygen, by their combination in carbonic acid (fixed air), 

 render the Grotto del Cane, at Naples, and the vapour 

 caverns in Pyrmont, torture-chambers for the poor dogs ; 

 lastly, nitrogen and hydrogen unite to form ammonia, the 

 volatile alkali, a kind of air which streams forth in enor- 

 mous quantity from those chimneys of subterranean fires 

 the volcanoes. Here we have the four elements, carbon, 

 hydrogen, oxygen and nitrogen, which, in combination, 

 form all those substances of which plants and animals are 

 composed ; hydrogen, oxygen, and nitrogen, being airs or 

 gases, carbon a solid substance, which in its crystalline form 

 we call diamond.* At the same time, too, we here men- 

 tion the most important and most generally diffused 

 compounds of these elements, namely, the usually fluid 

 water, which, however, is contained in large quantity by 

 the air, in the form of vapour ; also carbonic acid and 

 ammonia, both of which occur as gases in the atmosphere. 

 On the examination of these three compounds of these 

 four elements turns the whole study of vegetable and 

 animal life. 



Our atmosphere is a mixture of about of nitrogen 

 with | of oxygen, to which are added about ^-oVo part of 

 carbonic acid, and a yet undetermined quantity of ammonia. 

 Since we have, through Priestley, come to know oxygen, 

 and to comprehend its importance to respiration, we believe 

 that we are able to ascertain the goodness of air by deter- 

 mining the quantity of oxygen it contains. A peculiar 

 science, Eudiometry, has thus originated, which chiefly 

 concerns itself with the estimation of the relation of oxygen 

 and nitrogen in air ; the methods have gradually acquired 

 greater clearness and accuracy, and by these means it has 



* See Dumas and Boussingault, " The Chemical and Physiological 

 Balance of Organic Nature," 12mo. London, 1844. 



