2io ELEMENTARY SCIENCE 



oxygen is a part of air, they evidently have plenty of chance 

 to do this so far as the mere presence of oxygen is con- 

 cerned. But, fortunately for us, the combustion of most 

 inflammable substances requires more than the mere pres- 

 ence of air; it requires also more heat than is ordinarily 

 present. Once the process is started, however, it may of 

 itself generate enough heat to keep going until all the 

 near-by combustible material has been consumed. All the 

 fire departments and fire-insurance companies in the 

 world are results of this simple fact. It explains why, 

 when a fire gets started where we do not want it, we do 

 our best to put it out. 



You remember that the early scientists believed that 

 fire was due to the escape from burning substances of a 

 mysterious and invisible substance they called phlogis- 

 ton. This phlogiston theory, absurd as it may seem now, 

 has an important place in the history of science. It was 

 only a little more than a hundred years ago that it was 

 abandoned hi favor of the modern theory of combustion. 



The early scientists, though they misinterpreted the 

 process of burning, did recognize that it and the process of 

 rusting are similar processes. Iron-rust is iron plus oxygen 

 in the form of a substance called iron oxide. This substance 

 flakes away, and may finally disappear as dust, so we are 

 apt to regard rusting as a process in which something is 

 taken away from the iron. The iron does actually decay, 

 but the thing for us to remember is that rusting, and much 

 of what we call decay, are processes of oxidation, and that, 

 as such, they involve the addition of something rather than 

 the subtraction of something. In all kinds of combustion 

 the resulting substances may be or become gaseous or 

 liquid (iron-rust dissolves) and disappear, but the fact 



