COMBUSTION. 153 



is much above the ordinary temperature of the atmosphere, 

 so that our bodies are almost always giving out consider- 

 able heat to the air around us. This is very obvious where 

 there are many persons gathered together. A room that is 

 just comfortably warm with but few in it, becomes uncom- 

 fortably so very soon when it is crowded full of company. It 

 is very seldom that the air around us is as hot as our bodies, 

 and therefore very seldom that we are not giving off heat. 

 We are most comfortable when the air around us is at about 

 70 (Fahrenheit), and as this is 28 below the heat of our 

 bodies, we may say that we are comfortable only when we 

 are giving off considerable heat to the air. As this heat is 

 given off from the surface, the outer parts of the body are 

 not as warm as the inner. And as heat is constantly lost, 

 so it is constantly made. The myriads of furnaces are at 

 work all the time, night and day. The fires within us never 

 go out while life continues. 



200. Sources of the Fuel. The fuel used in producing 

 animal heat is carbon and hydrogen, as already intimated. 

 There are two sources from which these come : 1. The waste 

 of the body. In the wear and tear of the animal machine 

 there are particles every where that have ceased to-be use- 

 ful. They must be got rid of to make way for other par- 

 ticles to be deposited in their place. How is this done? 

 The oxygen that enters the lungs in breathing does it. This 

 goes in the blood to these useless particles, and burns up, 

 that is, unites with their carbon and hydrogen. This makes 

 heat just where the particles are, and the products of the 

 combustion, the carbonic anhydride and water, are carried 

 off in the blood that sweeps along in the veins. What be- 

 comes of them we will soon tell you. 2. A part of our food 

 furnishes fuel to feed the fires within us, the starchy, sug- 

 ary, and fatty articles. We shall speak particularly of this 

 subject in another part of this book, and so will not dwell 



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