WORLDS OF PHYSICS AND OF SENSE 103 



effort to invent the hypothesis that the water was the 

 same thing as the ice and snow, but in a new form. 

 Solid bodies, when they break, break into parts which 

 are practically the same in shape and size as they were 

 before. A stone can be hammered into a powder, but the 

 powder consists of grains which retain the character they 

 had before the pounding. Thus the ideal of absolutely 

 rigid and absolutely permanent bodies, which early physi- 

 cists pursued throughout the changing appearances, seemed 

 attainable by supposing ordinary bodies to be composed 

 of a vast number of tiny atoms. This billiard-ball view 

 of matter dominated the imagination of physicists until 

 quite modern times, until, in fact, it was replaced by the 

 electromagnetic theory, which in its turn is developing into 

 a new atomism. Apart from the special form of the 

 atomic theory which was invented for the needs of chem- 

 istry, some kind of atomism dominated the whole of 

 traditional dynamics, and was implied in every statement 

 of its laws and axioms. 



The pictorial accounts which physicists give of the 

 material world as they conceive it undergo violent changes 

 under the influence of modifications in theory which are 

 much slighter than the layman might suppose from the 

 alterations of the description. Certain features, however, 

 have remained fairly stable. It is always assumed that 

 there is something indestructible which is capable of motion 

 in space ; what is indestructible is always very small, but 

 does not always occupy a mere point in space. There is 

 supposed to be one all-embracing space in which the 

 motion takes place, and until lately we might have assumed 

 one all-embracing time also. But the principle of rela- 

 tivity has given prominence to the conception of "local 

 time," and has somewhat diminished men's confidence in 

 the one even-flowing stream of time. Without dogma- 



