70 ZOOLOGY. 



to it by being exposed to the action of the air whilst tra- 

 versing in vessels an organ adapted for this purpose. This 

 process of aeration is called respiration. The necessity for 

 this is proved by the simple experiment, if any such were 

 wanted, of placing an animal under the receiver of an air- 

 pump, and exhausting the air ; in a certain time the animal 

 dies asphyxiated. Wherever there is life, whether animal or 

 vegetable, air is essential. The term applies equally to aquatic 

 animals, which live by means of the air held in a kind of 

 solution or mixture by the waters in which they exist. 



117. The air we breathe, and which is essential to all 

 that lives, is a compound fluid. It is composed of, 1. Watery 

 vapour, always present in greater or less quantities ; and 2. 

 In 100 parts of pure atmospheric air : there are 20 of oxygen, 

 79 of azote or nitrogen, with some traces of carbonic acid 

 gas. Oft-repeated chemical experiments have proved that it 

 is the oxygen alone which maintains life. The discovery of 

 this singular fact we owe (1777) to Lavoisier, a celebrated 

 French chemist, who was barbarously executed during the 

 French Eevolution. 



118. By the action of respiration the oxygen is with- 

 drawn from the atmosphere, and disappears ; in its place we 

 find carbonic acid gas; this gas is not respirable, i. e., if 

 breathed it destroys life. In the consumption of oxygen and 

 the production of carbonic acid gas, respiration essentially 

 consists. 



119. Although azote or nitrogen be not a vital gas, its 

 presence in the atmosphere seems necessary to dilute the 

 oxygen : and it has been observed that a certain quantity of 

 azote is absorbed and given out in the act of respiration. 



120. Finally, there escapes with the fluids expired a 

 certain amount of vapour, which becomes conspicuous in cold 

 weather ; this vapour is called pulmonary transpiration. 



121. It is whilst passing through the capillaries of the 

 lungs that the blood loses its dark venous hue, and becomes 

 of a bright vermilion arterial colour. Many experiments 

 have been made to prove that which seems obvious without 

 any. 



122. Theory of Respiration. What becomes of the 

 oxygen which has disappeared, and what is the source of the 

 carbonic acid gas uniformly found as a product of respiration ? 

 The strong analogy existing between the phenomenon of 

 combustion, of a piece of charcoal for example, and respira- 



