16 CHEMICAL COMPOSITION OF LIVING MATTER 



Carbon. — (Sec Ex. V.) When we partly burn a piece 

 of wood, bone, or flesh, we find it transformed into a 

 black substance which we call charcoal. In nature we find 

 this same substance in a much harder form and we call 

 it coal. When we use our lead pencils the mark we make 

 is due to another form of this substance called graphite. 

 Finally in the diamond we have a substance very different 

 in appearance from charcoal, coal, or graphite, but essen- 

 tially like them in some respects. All of these substances 

 are forms of a very abundant chemical element called 

 carbon. So important a part of all living matter is this 

 element that the study of its compoimds is made a special 

 branch of chemistry under the name Organic Chem- 

 istry. 



In most of its forms, carbon shows a black color and is 

 without odor or taste. In its free state it has so little 

 affinity for other elements that farmers often char the ends 

 of fence posts before setting them in order to insure against 

 decay. When heated to a certain temperature, however, 

 all forms of carbon burn and form a colorless gas, which 

 may be readily detected by its power to turn clear lime 

 water milky. This gas is a chemical compound of carbon 

 with the oxygen of the air and is called carbon dioxide. It 

 is this gas that is used to charge water in the manufacture 

 of ordinary soda water. Just as in the union of phos- 

 phorus and sulphur with oxygen, the formation of this 

 gas is accompanied by the generation of much heat, and 

 this fact is made use of in our bodies as well as in our 

 stoves and furnaces. If we blow our breath through clear 

 lime water the milky color that results gives unmistakable 

 evidence that this gas is being constantly formed in our 

 bodies and given off with the exhaled air. 



