262 THE GRAMMAR OF SCIENCE 



a possible simplification of our scientific analysis of 

 phenomena that we must devote a few pages to its 

 discussion. We will term the fundamental element of 

 heavy-matter, the element out of which, perhaps, chemical 

 atoms themselves are to be conceived as built up, the 

 prime-atom. We have, then, to ask what types of motion 

 in the ether have been suggested as possible forms for the 

 prime - atom. There are two suggestions to which 

 reference may be made, both of which depend upon our 

 postulating the same constitution for the ether. We 

 must here make a brief digression in order to throw some 

 light on this constitution of the ether. 



§ 9. — The Ether as ''Perfect Fluid'' and " Perfect Jelly'' 



The reader is certainly acquainted with two types of 

 perceptual bodies which may be roughly described as 

 liquid and elastic. As specimens of these two types we 

 will take water and jelly. As substances water and jelly 

 have a remarkable agreement in one respect and a 

 remarkable divergence in another. If we put either water 

 or jelly into a cylinder closed at the bottom and attempt 

 to compress them by aid of a heavily-loaded piston, we 

 shall find that the compression is either insensible or of 

 very small amount indeed. Careful experiments with 

 elaborate apparatus show that these substances are com- 

 pressible, but the amount of compression, although 

 measurable, is exceedingly minute as compared, for 

 example, with the amount that air would be compressed 

 by the same load. We express this result by saying that 

 both water and jelly offer great resistance to one form of 

 strain, namely, change of size (p. 203). But this resist- 

 ance is only relative, relative to other substances, such as 

 gases, and to the machinery of compression at our 

 disposal. So far as our perceptive experience goes, there 

 is no substance which resists absolutely all change of size, 

 or for which change of size is impossible. Hence an 

 incompressible substance is merely a conceptual limit 

 which has not its equivalent in the world of phenomena. 



