132 RESPIRATION. 



15. Changes in the Air from Respiration. Air 



that has been once breathed is no longer fit for respiration. 

 An animal confined within it will sooner or later die; so 

 too, a lighted candle placed in it will be at once extin- 

 guished. If we collect a quantity of expired air and ana- 

 lyze it, we shall find that its composition is not the same 

 as that of the inspired air. When the air entered the 

 lungs it was rich in oxygen ; now it contains twenty-five 

 per cent, less of that gas. Its. volume, however, remains 

 nearly the same ; its loss being replaced by another and 

 very different gas, which the lungs exhaled, called carbonic 

 acid, or, as the chemist terms it, carbon dioxide. 



16. The expired air has also gained moisture. This is 

 noticed when we breathe upon a mirror, or the window- 

 pane, the surface being tarnished by the condensation of 

 the watery vapor exhaled by the lungs. In cold weather, 

 this causes the fine cloud which is seen issuing from the 

 nostrils or mouth with each expiration, and contributes in 

 forming the feathery crystals of ice which decorate our 

 window-panes on a winter's morning. 



17- This watery vapor contains a variable quantity of 

 animal matter, the exact nature of which is unknown; but 

 when collected it speedily putrefies and becomes highly 

 offensive. From the effects, upon small animals, of con- 

 finement in their own exhalations, having at the same 

 time an abundant supply of fresh air, it is believed that 

 the organic matters thrown off by the lungs and skin 

 are direct and active poisons; and that to such emana- 

 tions from the body, more than to any other cause, are 

 due the depressing and even fatal results which follow the 

 crowding of large numbers of persons into places of lim- 

 ited capacity. 



1 !i. Air once breathed ? An animal in it ? A candle ? Analysis of expired air ? 

 Change in volume ? 



1 6. What else has the expired air pained ? When and where noticed ? 



17. Nature of the watery vapor ? Its effects upon animals ? 



