Minor Constituents of Air 227 



expired air. They have already been shown to be 

 the products of combustion of substances containing 

 carbon and hydrogen. Now bread, meat, vegetables, 

 milk, cheese, and all other food-stuffs contain carbon 

 and hydrogen, and by their slow oxidation during the 

 vital processes two of the chief bodies produced are 

 water and carbon dioxide. (For a method of showing 

 the presence of carbon and hydrogen in food-stuffs, see 

 Part II.) 



155. In the preceding chapter it was shown that 

 chemical combination is usually attended with the 

 evolution of heat. That heat is given out during the 

 oxidation of the constituents of our food is rendered 

 evident by the fact that our bodies maintain an almost 

 even temperature, although our surroundings are 

 usually at a temperature considerably below that of 

 the body. The heat given out by the oxidation is 

 used to maintain this difference of temperature between 

 our bodies and their surroundings. 



156. It was stated in paragraph 149 that the pro- 

 ducts of the burning of a candle weighed more than the 

 candle itself. To test this, the following experiment 

 may be performed. 



EXPERIMENT 90. Arrange the apparatus shown in 

 Fig. 72. 



Before beginning the experiment, weigh the candle, 

 chimney, and U-tube with their connections and corks. 

 Attach this weighed portion of the apparatus to the 

 aspirator, and turn the stopcock so that a fairly rapid 

 stream of water issues. This will cause a steady 

 current of air to pass up the chimney and through the 

 U-tube into the aspirator. Remove the candle, light 

 it, and quickly replace cork and candle in position as 

 in the diagram. Arrange the rate at which the current 



152 



