THOUGHTS ON NATURAL PHILOSOPHY. 13 
weight, give to substances their distinguishing characteristics, 
and account for all natural phenomena. 
Illustrations are dangerous things in an argument; but, if 
apposite and not unduly pressed, they may be helpful. 
Start one hundred men on a foot race of a mile. Soon 
they will fall into groups, as a result of the different rates of 
speed at which they travel; and these groups will possess 
mobility and stability dependent on the speed and movement 
of the individuals forming the groups. 
Imagine an immense billiard table and a vast 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 interfered 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 energised 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. 
*Although they may kindle more or less slowly at different 
temperatures the truth remains. 
