THE ATMOSPHERE 199 



separated " dephlogisticated air " (oxygen), by heating with a burn- 

 ing-glass red-lead over mercury. He determined that " fixed 

 air" (C0 2 ), is a product of respiration, and that plants purified 

 confined air and made it again fit for respiration. Priestley de- 

 termined that combustion, respiration, and the colour of arterial 

 blood depended on the presence of " dephlogisticated air." But 

 confined by the theories of Stahl, he misinterpreted his results, and 

 supposed that respiration disembarrassed the body of " phlogiston " 

 or fire matter introduced with the food, that arterial blood 

 became red because it was freed from " phlogistic air " (nitrogen). 

 Lavoisier (17431794), hearing of Priestley's discovery, by his 

 memorable experiments lasting twelve days successively synthe- 

 sised and analysed red oxide of mercury, showed that this is a 

 combination of oxygen and mercury, and that the separation 

 of the two is effected by heat. Stahl and his school supposed 

 the metals to be compounds of earths charged with the matter 

 of fire, impregnated with " phlogiston " ; that the earths were 

 simple bodies freed from phlogiston by calcination. In the hands 

 of Lavoisier the process of oxidation became clear, and combus- 

 tion was proved to be a case of oxidation. 



Air is not a chemical compound but a mixture. Analysed in 

 the dry state, it yields on the average by volume 20'93 per cent. 

 2 , 79*04 per cent. N 2 , -03 per cent. C0 2 with traces of ammonia 

 and ozone, argon, krypton, neon, metargon. The last four gases 

 owe their presence to their non-combining property. They are 

 residues not employed in the building of the earth's crust. 

 Air, when pure, is free from colour, taste, and smell. It has fre- 

 quently been asserted that air varies slightly in composition at 

 different places and seasons. Such variations are, with the ex- 

 ception of C0 2 , due to unavoidable errors in analysis errors 

 which are greater, or smaller, according to the degree of perfec- 

 tion of the analytical apparatus and the skill of the analyst. 



Air collected by means of balloon sounds at an altitude of 

 15,000 m. has the same composition as air at sea-level. Air 

 taken from the ill- ventilated pit of a theatre contained 20' 7 

 per cent. 2 by volume. In towns the C0 2 percentage may rise 

 to 6-9 parts per 10,000. Ozone contains three atoms of 

 oxygen in its molecule, one being in the active state. The traces 

 of it formed by atmospheric electricity are more abundant on 

 mountains and at the seaside. 



