SPACE AND TIME 175 



squeeze iron or granite are capable of measurement. 

 Now it is very hard, I think impossible, to conceive how 

 we can alter the size of bodies if we suppose them 

 continuous. We feel ourselves compelled to assert that, 

 if the parts of a body move closer together, they must 

 have something free of body into which they can move. 

 If a body were continuous and yet compressible, there 

 appears to be no reason why it should not be indefinitely 

 compressible, or indefinitely extensible, both results re- 

 pugnant to our experience. Further, our sense-impres- 

 sions of temperature in both gaseous and solid bodies, 

 and of colour in solid bodies, the phenomena of pressure in 

 gases, and those of the absorption and emission of light, 

 are easily analysed and described, if we conceive the 

 ultimate parts of bodies to have a capacity for relative 

 motion ; but there is no possibility of conceiving such a 

 motion if all the parts of a body are continuous. A 

 crowd of human beings seen from a great height may 

 look like a turbulent fluid in motion at every point. But 

 we know from experience that this motion is only possible 

 because there is some void in the crowd. It may become 

 so densely packed that motion is no longer practicable. 

 Thus it is with that relative motion of the parts of 

 bodies upon which so much of modern physics depends ; 

 absolutely close packing, that is continuity, seems to 

 render it impossible. It is only by reducing in conception 

 the complex groups of sense-impressions, which we term 

 bodies, into simple elements directly depending on the 

 motion of discontinuous systems, — of what we may term 

 granular or starlike systems, — that we have been able to 

 resume phenomena in the wide-reaching laws of physics 

 and chemistry. The relative motion of the ultimate 

 parts of bodies, involving the idea of discontinuity, is one 

 of the fundamental conceptions of modern science (p. 133). 

 These ultimate parts of bodies we are accustomed to 

 speak of as atoms; groups of atoms which apparently 

 repeat themselves over and over again in the same body — • 

 something like planetary systems in the starry universe — 

 we term molecules. The generally accepted atomic or 



