COMBUSTION. 155 



because the circulation is quickened, and with it the breath- 

 ing, and so more of the oxygen is introduced into the blood, 

 and thus to the carbon and hydrogen- The same effect is 

 produced in this way upon the combustion of the body as 

 upon an ordinary fire by blowing it. It is simply increasing 

 one of the three requisites for combustion mentioned in 194. 



204. Cold-blooded Animals. Reptiles and fishes are cold-blooded 

 animals that is, they have nearly the same temperature with the medium 

 in which they live. The fires in them are not at all brisk, and they use 

 little oxygen in comparison with warm-blooded animals. They have need 

 of but little, for they live a comparatively sluggish life, as you may fully 

 realize in relation to reptiles if you observe the difference in activity be- 

 tween a bird and a frog or toad. It may appear to you that this is not 

 true of fishes, as their motions are often very quick. But it must be re- 

 membered that it requires but little exertion really for them to move with 

 rapidity, because they live in a medium of a specific gravity so near their 

 own. For further illustration of this point we refer you to Chapter XX. of 

 Hooker's " Natural History." 



205. Hibernation. Animals are said to hibernate who go into a 

 torpid state in the winter. The degree of torpidity varies much in differ- 

 ent animals. In cold-blooded animals respiration and circulation may 

 cease altogether, and the operations of life may be as thoroughly suspend- 

 ed as in a seed that is kept from heat and moisture. They may be pre- 

 served in this state for a long time. Frogs and serpents have been kept in 

 ice for years without any signs of life, and then have been revived by ex- 

 posure to a warm atmosphere. While animals are in such a state, the ma- 

 chinery of life being stopped, there is no wear and tear, and therefore no 

 waste to be got rid of. As there is nothing to bum, no oxygen is needed. 

 In hibernating warm-blooded animals the torpidity is not so thorough, and 

 in proportion to the movements of life there is waste, and therefore need of 

 oxygen to burn it. Hence there is occasional respiration. In such cases 

 of imperfect torpidity the fat which has been acquired in summer is burned 

 up for the purpose of maintaining the requisite warmth, and such animals 

 therefore come out in the spring from their hiding-places in quite a lean 

 condition. 



206. The Chief Elements. The four elements with which 

 you have become so familiar in the previous chapters 



