142 NON-METALS AND THEIR COMBINATIONS. 



called oxidizing agents, as, for instance, nitric acid, potassium chlorate, 

 potassium permanganate, etc. 



In all combustions we have at least two substances acting chemically upon 

 one another, which substances are generally spoken of as combustible bodies 

 and supporters of combustion. Illuminating gas is a combustible substance, 

 and oxygen a supporter of combustion ; but these terms are only relatively 

 correct, since oxygen may be caused to burn in illuminating gas, whereby it is 

 made to assume the position of a combustible substance, while illuminating gas 

 is the supporter of combustion. 



While some substances, such as iron and phosphorus, undergo slow combus- 

 tion at the ordinary temperature, there is a certain degree of temperature, 

 characteristic of each substance, at which it inflames. This point is known as 

 kindling temperature, and varies widely in different substances. Zinc ethyl 

 ignites at the ordinary temperature, phosphorus at 50 C. (122 F.), sulphur at 

 about 450 C. (842 F.}, carbon at a red heat, and iron at a white heat. The 

 heat produced by the combustion is generally higher than the kindling tem- 

 perature, and it is for this reason that a substance continues to burn until it is 

 consumed, provided the supply of oxygen be not cut off, and the temperature 

 be not through some cause lowered below the kindling temperature. 



The total amount of heat evolved during the combustion of a substance is 

 the same as that generated by the same substance when undergoing slow com- 

 bustion, but the intensity depends upon the time required for the oxidation. 

 A piece of iron may require years to combine with oxygen, and it may be 

 burned up in a few minutes ; yet the total heat generated in both cases is the 

 same, though we can notice and measure it in the first instance by most deli- 

 cate instruments only, while in the second it is very intense. 



While heat is evolved when two or more elements combine chemically, heat 

 is absorbed when decomposition takes place. In fact, the quantities of heat 

 evolved and absorbed by combining and decomposing identical quantities of 

 matter are absolutely alike. Thus, heat is evolved when mercury and oxygen 

 combine, but the same quantity of heat .is absorbed when the mercuric oxide 

 thus formed is decomposed into its elements by the action of heat. 



Whenever a substance has the power to unite with others, it can do chemical 

 work ; it possesses chemical energy. Consequently, all combustible substances 

 can do work ; i. e., by combining with oxygen they evolve heat, which in turn 

 may be transformed into motion or into some other form of energy. 



The chief supply of chemical energy at our disposal is derived from plant- 

 life. All kinds of wood, and its decomposition-product, coal, possess chemical 

 energy. This energy is stored up in vegetable matter, because the sun's heat 

 caused a decomposition of water and carbon dioxide, which substances are the 

 two chief compounds used in the construction of plant tissue. In burning 

 vegetable matter the oxygen removed from the water and carbon dioxide by 

 the action of the sun's rays is taken up again, and heat is evolved. 



Ozone is an allotropic modification of oxygen, which is formed 

 when non-luminous electric discharges pass through atmospheric air 

 or through oxygen ; when phosphorus, partially covered with water, 



