SCIENCE PRIMERS, [§ i. 



into ashes, carbonic acid, and water (with one or 



two other things of which we shall speak afterwards). ^ 



Your body is always giving out carbonic acid , ^ 



(Chemistry Primer, Exp. 7). Your body is always 

 giving out water by the lungs, as seen when you 

 breathe on a glass, by the skin, and by the kidneys ; 

 and we shall see that we always give out more 

 water than we take in as food or drink. Your body 

 too is daily giving out by the kidneys and bowels, 

 matters which are not exactly ashes, but very like 

 them. We do not oxidize our food quite into ashes, 

 but very nearly; we burii it into substances which 

 are no longer useful for oxidation in the body, and 

 which, being useless, are cast out of the body as 

 waste matters. 



The tale then is complete. By the help of the 

 oxygen of the air which you take in as you breathe, 

 you oxidize the food which is in your body. You get 

 rid of the water, the carbonic acid, and other waste 

 matters which are left after the oxidation, and out of 

 the oxidation you get the heat which keeps you warm 

 and the power which enables you to move. 



Thus all your life long you are in constant need of 

 oxygen and food. The oxygen you take in at every 

 breath, the food at every meal. How you get rid of 

 the waste matters we shall see further on. 



If you were to live, as one philosopher of old 

 did, in a large pair of delicate scales, you would find 

 that the scale in which you were would sink down at 

 every meal, and gradually rise between as you got 

 lighter and hungry. If the food you took were more 

 than you wanted, so that it could not all be oxidized, it 

 would remain in your body as part of your flesh, and 



