II. HEAT IN RELATION TO THE HUMAN BODY 



COMBUSTION 



For a complete understanding of the changes known as 

 combustion one must study Chemistry. Combustion com- 

 monly means oxidation, i.e., union with oxygen (O), accom- 

 panied by noticeable heat and light as result of the chemical 

 change. Coal is largely carbon (C), and when it burns 

 (oxidizes) one of the products is a gas known as a carbon dioxide 

 whose chemical formula is CO 2 . When the gas hydrogen (H) is 

 burned, the product is water (H 2 O). Paraffine, tallow, wood, 

 kerosene, and many other substances used for fuel and 

 lights, contain carbon and hydrogen in their chemical com- 

 position. When burned they yield both water and carbon 

 dioxide as the principal products of their combustion. Oxy- 

 gen exists free in the air mixed with about four times its 

 volume of nitrogen. 



Oxygen as an element in water is so much heavier than 

 hydrogen that eight-ninths by weight of all water is oxygen. 

 It is calculated that oxygen constitutes about one-half the 

 weight of the chemical compounds forming the rocks of the 

 earth's crust. 



One of the most important teachings of chemistry is that 

 the same substance is always made up of the same chemical 

 elements, and that these are always united in the same propor- 

 tions by weight. The compound known as water has the 



16 



