80 THE PRODUCTION AND USE OF HEAT 



salt. Carbon and oxygen may be united to form CARBON 

 DIOXIDE, a gas. In other proportions carbon and oxygen may 

 be united to form a different compound, known as CARBON 

 MONOXIDE. Hydrogen and oxygen unite to form HYDROGEN 

 OXIDE (WATER). In other proportions hydrogen and oyxgen 

 unite to form HYDROGEN DIOXIDE (HYDROGEN PEROXIDE) com- 

 monly called "peroxide." 



89. Chemical Compounds do not Vary in Composition. 

 Chemical elements, not only unite to form compounds with the 

 accompanying chemical change, but, moreover, the elements 

 are found in the compound in certain fixed and definite propor- 

 tions by weight. Thus it is found that the compound copper 

 sulphide is very nearly % sulphur and % copper by weight. 

 Common salt (sodium chloride) is composed of nearly % chlorine 

 and % sodium. Carbon dioxide is composed of just %i carbon 

 and %i oxygen while carbon monoxide is composed of just ^ 

 carbon and % oxygen. Water is nearly J-^J hydrogen and % 

 oxygen while hydrogen peroxide is >{ 7 hydrogen and l % 7 

 oxygen. Each chemical compound has unvarying composition. 



90. Oxygen and Getting Energy from Ffyels. Oxygen com- 

 bines with nearly every other element, but with some more 

 readily than with others. When it combines with the fuel 

 elements, carbon and hydrogen, it liberates energy and forms 

 carbon monoxide or carbon dioxide with the carbon, and water 

 with the hydrogen. These compounds are the common prod- 

 ucts arising from the burning of fuels. Oxygen is necessary 

 for burning all fuels. In a certain sense, it is the element which 

 liberates the chemical energy bound up in fuels, thus making 

 the fuel useful to man. The oxygen for burning fuels comes 

 from the air. 



91. Constituents of the Air. Air is a mixture of gases, con- 

 sisting of the elementary gases, oxygen and nitrogen, and the 

 chemical compounds, water vapor and carbon dioxide. Besides 

 these there are several other gaseous elements and compounds 

 in the air but they are unimportant for our purpose. The air 

 usually contains tiny particles of solids as dust and germs. 



