186 RESPIRATION 



the chemist as oxygen and nitrogen, in the proportion of one 

 part of the former to four parts of the latter. These gases are 

 very unlike, being almost opposite in their properties : nitrogen 

 is weak, inert, and cannot support life : while oxygen is power- 

 ful, and incessantly active, and is the essential element which 

 gives to the atmosphere its power to support life and com- 

 bustion. The discovery of this fact was made by the French 

 chemist, Lavoisier, in 1778. 



15. Changes in the Air from Respiration. Air that has been 

 once breathed is no longer fit for respiration. An animal con- 

 fined within it will soon die ; so, also, a lighted candle placed 

 in it will be at once extinguished. If we collect a quantity of 

 expired air and analyze 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 made up by another 

 and very different gas, which the lungs exhale, called carbonic 

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



16. The expired air has also gained moisture. This is no- 

 ticed when we breathe upon a mirror or the window-pane, the 

 surface being tarnished by the condensation of the watery vapor 

 given off 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 ani- 

 mal 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 confinement in their 

 own exhalations, having at the same time an abundant supply 

 of fresh air, it is believed that the organic matters thrown off 

 K v the lungs and skin are direct and active poisons ; and that 



15. Air once breathed ? An animal in it ? A candle ? Analysis of expired air ? 

 Change in volume ? 



16. What else has the expired air gained ? When and where noticed ? 



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



