LESSONS IN CHEMISTRY 17 



different individuals, but it is always present in some quan- 

 tity. Muscle, however lean, contains particles of fat; fat 

 constitutes a small percentage of the blood; it fills the 

 spaces in the interior of bones ; and it is often deposited 

 in considerable quantity in the deeper layers of the skin. 

 When fat is heated, it first melts to a liquid. At a higher 

 temperature it will scorch, and the black residue shows the 

 presence of carbon. In the body this fat is burned by com- 

 bining with oxygen, and this is one of the ways in which we 

 are kept warm. If we were to eat nothing for several days, 

 we could still be kept warm and be able to do a certain 

 amount of work, a result due largely to the slow oxidation 

 of the fat stored in various parts of the body. 



Carbohydrates. The substances we know as starches and 

 sugars are made up of the three chemical elements, carbon, 

 hydrogen, and oxygen, and the hydrogen and oxygen of 

 these compounds are always in the same proportion in 

 which they occur in water (that is, H 2 0). Hence these 

 compounds are called car-bo-hy' drates (carbon +hy dor = 

 water). In the blood and other animal tissues we find 

 some of the carbohydrate called grape sugar. 1 Another 

 carbohydrate, known as animal starch or glycogen, is found 

 In the liver. 2 Carbohydrates, like fats, contain a large 

 amount of carbon, which also unites with oxygen. A sec- 

 ond ingredient of both of these classes of compounds is 

 hydrogen ; it readily combines with oxygen to form water. 

 The fats and carbohydrates found in the composition of the 

 body may be regarded as stored-up fuel which can be drawn 

 upon in case of need. Like the engine, we are kept warm 

 and enabled to do work by the oxidation of fuel. 



1 The chemical composition of grape sugar is represented by the 

 chemical formula C 6 Hi 2 6 , which means that every molecule of grape 

 sugar contains six atoms of carbon, twelve atoms of hydrogen, and 

 six atoms of oxygen. 



2 Glycogen is comp6sed of six atoms of carbon, ten atoms of hydro- 

 gen, and five atoms of oxygen, its chemical formula being C 6 H 10 5 . 



c 



