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combustion begins involve the physical condition of the solid, the 

 pressure of the gas or vapor, the presence or absence of a contact 

 agent and the nature of the contact agent, as well as the temper- 

 ature. No definite kindling temperature can be given, unless the 

 other conditions are specified also. Kindling conditions involve 

 several variables, of which the temperature is only one. 



Oxidation. The slower union with oxygen which occurs in 

 rusting is called oxidation. We shall see later, however, that it 

 has been found convenient to stretch this term so as to cover com- 

 binations of other elements than oxygen, and even to include 

 actions not involving combination. At this point we can dis- 

 cuss only oxidation by oxygen. 



This process of slow oxidation by oxygen, although less con- 

 spicuous than combustion, is really of greater interest. Thus the 

 decay of wood is simply a process of oxidation whereby the same 

 products are formed as by the more rapid ordinary combustion. 

 Sewage is mixed with large volumes of river water, the object 

 being, not simply to dilute the sewage, but to mix it with water 

 containing oxygen in solution. This^has an oxidizing power like 

 that of oxygen gas and, through the agency of bacteria, quickly 

 renders dissolved organic matters innocuous by converting them 

 for the most part into carbon dioxide and water. Thus, a few 

 miles further down the stream, the water may become as suitable 

 for drinking as it was before the sewage entered. In our own 

 bodies we have likewise a familiar illustration of slow oxidation. 

 Avoiding details, it is sufficient to say that the oxygen, from the 

 air taken into the lungs, combines with the haemoglobin in the red 

 blood-corpuscles. In this form of loose combination, it is carried 

 by the blood throughout our tissues and there oxidizes the food- 

 stuffs which have been absorbed during digestion. The material 

 products are carbon dioxide and water, of which the former is 

 carried back to the lungs by the blood, and finally reaches the 

 air during exhalation. The important product, however, is 



