36 THE MECHANISM OF LIFE 



the same. Take a piece of wire of uniform diameter and density, 

 and (say) 10 inches long, and put it in one scale, and put a piece of 

 the same wire 1 inch long in the other. The mass of the 10- inch 

 length will be greater than that of the 1-inch piece, for the scale 

 pan in which it is placed will fall, and the other one will rise. 

 Put a 1-inch length in one pan and another 1-inch length in the 

 other, and it will be found that the work done by the earth's 

 gravity is the same in each case, and the masses are therefore 

 equal. Finally, put the ten 1-inch pieces in one pan and the 

 10- inch piece in the other, and again it will be found that the work 

 done is the same in each case, so that the mass of each 1-inch 

 length is one-tenth of that of the 10-inch length. 



Thus we can find the mass of a body by measuring the work 

 done upon it, when it is free to fall, by the earth's gravity, and 

 comparing this with some standard amount of work done. In 

 whatever way we measure this work done it will be found that 

 I we always measure a space. Even when we measure time it is 

 really a space that we determine. 



A material body that is in a state of rest will continue in a 

 state of rest, or if it is in a state of uniform motion it will continue 

 t in that state of motion unless work is done upon it. Of itself it 

 is inert. Its inertia varies according to its mass, so if a greater 

 mass that is at rest is to be moved, or if a greater mass that is in 

 uniform motion is to be stopped, a greater amount of work must 

 be done. Inertia means the tendency of something to remain as 

 it is, unless some external agency acts upon it. 



Modern Theories of Matter. 



We must say a few words about these. Not so long ago it was 

 thought that all material bodies, or kinds of matter, were built up 

 of chemical atoms which were the ultimate particles. An atom 

 .was regarded as indivisible. But some atoms were known to be 

 heavier than others, and so their masses were not the same ; thus 

 the mass of an atom of platinum is about 194 times greater than 

 that of an atom of hydrogen. Therefore, the heavier atoms had 

 more of something in them than the lighter ones, but more of 

 what ? There might be some universal kind of matter contained 

 in greater quantity in the heavier than in the lighter atoms, but 

 if so the former could hardly be thought about as indivisible, 

 which they must be, according to chemical theory. 



The way out from this paradox was found by the discovery of 



