OXYGEN 17 



A study of the element carbon demonstrates three 

 important facts. 



First. It is present in large amounts as a part of all 

 living matter. 



Second. Its combination with oxygen to form carbon 

 dioxide is always accompanied by the production of heat, 

 and the formation of this gas in our bodies results in main- 

 taining a constant body temperature. 



Third. A chemical element in its free state may occur 

 in forms which appear very different to the eye, and must 

 be tested by other means than sight to determine its 

 identity. 



Oxygen. — (See Exs. VII. and VIII.) This important 

 element occurs free in the air in the form of a colorless, 

 odorless, and tasteless gas. In its combined form it is 

 abundant in both living and non-living matter. It is of 

 great importance to all living processes because it has a 

 great affinity for other elements, and because, in combin- 

 ing with other elements, it sets free a great amount of 

 energy in the form of heat. 



T^Tiile oxygen is present in the air in a free state, it is 

 so mixed with other gases that special processes must be 

 resorted to to obtain it pure. The simplest method of 

 obtaining oxygen is to heat a compound which contains 

 it. Some of these compounds when so heated break up 

 and set free the pure gas. Two compounds which are 

 especially favorable to this method are oxide of mercury 

 (a compound of mercury and oxygen), and potassium chlo- 

 rate (potassium, chlorine, and oxygen). 



If we heat some oxide of mercury in a tube, as shown 

 in Figure 1, we note that it gradually disappears. If, 

 while the heating is going on, we insert the glowing end 



EDDY. PHYS. — 2. 



