52 THOUGHTS ON NATURAL PHILOSOPHY. 



number of billiard balls rolling about the table ; the 

 balls will fall into groups according to the speed and 

 movement of the individual balls, and those groups 

 will have a sort of mobility and stability until inter- 

 fered with by other groups or balls crashing against 

 them with sufficient force and weight to break up the 

 group, with the result that new groups are formed. 

 What is true in this respect of masses of matter, the 

 size of men or billiard balls, is true of smaller and 

 larger portions of matter. 



A long strip of wood usually burns slowly. The 

 reason for this is that only the particles of those parts 

 of the wood nearest the burning part become energized 

 to move with sufficient speed to combine with the 

 oxygen. The particles of the unburning portion of 

 wood are moving too slowly to unite with the oxygen ; 

 if they were moving at sufficient speed to combine with 

 the oxygen, the wood would be burnt up in a flash. 



Every combustible substance has its * kindling 

 temperature, at which its particles move' at sufficient 

 speed to combine with the oxygen. 



One of the most difficult problems in connection 

 with this subject is the following : Why is it that in 

 combustion, although the particles must move at 

 varying rates of speed, only the products (and the 

 same products each time) with which we are ac- 

 quainted are produced ? The explanation appears to 

 be as follows : Imagine a box of billiard balls 

 revolving rapidly, and the balls suddenly thrown 

 therefrom at high rates of speed on to a very large 



* Although they may kindle more or less slowly at different 

 temperatures, the truth remains. 



