218 



COMMON SUBSTANCES 



G-unpowder is a mixture of saltpeter (KNO 3 ), carbon, 

 and sulphur. Upon exploding, the gases N and CO 2 

 are set free. Gf-un cotton is a nitrate of cellulose chiefly. 

 Glycerin also unites with HNO 3 , forming a nitrate 

 known as nitroglycerin. Nitroglycerin is a pale yellow 

 liquid, highly explosive. It is used in making dynamite 



and some other explo- 

 sives. Note that these 

 substances all contain 

 some nitrogen; because 

 N is so inactive it forms 

 compounds that easily 

 break up and set the 

 N (gas) free. 



FIG. 143 



Experiment 137. In 



a mortar mix 12 grams 

 KNO 3 , 2 g. of C (charcoal), 

 and \% g. of S. When thoroughly mixed, put a small amount 

 on a piece of metal and touch it with a lighted match. Notice 

 how it burns. The mixture is gunpowder. Why does it not 

 explode with a loud report ? Try to burn some gunpowder by con- 

 verging the sun's rays upon some one spot, as in Experiment 94 

 (see Fig. 143). 



258. Foods. Animal bodies are made up of the same 

 elements that compose matter in general, and only a 

 few of these elements are present in any quantity. 

 Since animals grow by taking in food, we can get a 

 good idea of what elements are most needed by study- 

 ing the foods used. Man is supplied with food that is 

 largely of either animal or plant growth ; but since the 

 animals eat either plants or other animals that may live 



