THE LEAF 141 



insignificant impulse to start them ; those of the latter 

 kind on the contrary require an external supply of energy, 

 which can be absorbed in the process. 



Most of the phenomena of chemical combination fall 

 under the former of the two categories ; most of the 

 phenomena of chemical decomposition fall under the 

 latter. Burning, i.e. combining with oxygen, may serve 

 as the simplest example of chemical combination. The 

 converse phenomenon of unburning, the withdrawal of 

 oxygen from a body, which is called reduction, may 

 serve as the simplest example of decomposition. 



We can try to demonstrate what takes place during 

 chemical combination and decomposition, i.e. during 

 combustion and reduction, by means of the following 

 simple model consisting of two little balls of lead at 

 the end of thin threads (fig. 41). 



The cause of any chemical combination lies in the 

 fact that chemical substances are endowed with a 

 tendency, a kind of gravitation, towards each other. 

 We call that tendency chemical affinity. Particles of 

 carbon and oxygen tend towards each other in the way 

 these balls a and & do if I 

 move them apart and leave 

 them to themselves. But 

 we know that the impact / 



of one body against another / 

 produces heat and some- ^ a 

 times light. Heat and 



light developing from this a/ 



impact, i.e. from invisible 



collisions between particles FIG. 41. 



of carbon and hydrogen 



and particles of oxygen, are indeed the heat and light 



that we observe in a burning flame. 



We thus perceive the cause of spontaneous chemical 

 combination, and the reason why it is accompanied 

 by the development of heat. In combining, chemical 



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