MATTER, SPACE, AND TIME 205 



processes confined to fluids. Sir William Roberts- 

 Austen has shown that gold, If placed in intimate 

 contact with lead, will diffuse at ordinary tempera- 

 tures to such an extent that, after the lapse of 

 some years, it can be detected in the lead by 

 chemical analysis at distances of a millimetre or 

 more from the surface of contact. Chemical 

 analysis is by no means one of the most sensitive 

 methods of research, and to be discovered in 

 this way the gold must have migrated to a con- 

 siderable extent. 



Again, our usual conceptions of the nature 

 of heat rest on the view that it is a form of 

 energy — the energy of agitation of particles too 

 small to be recognised or controlled individually 

 by ordinary mechanical means. No hypothesis 

 previously proposed fulfils the needs of a satis- 

 factory explanation. 



While the most obvious phenomena thus point 

 consistently to the conception of the grained 

 structure of matter, the more recondite branches 

 of physical science indicate the same conclusion 

 by evidence which, In its cumulative effect, is 

 irresistible. The phenomena of liquid electrolysis, 

 no less than those of gaseous discharge and radio- 

 activity, have been successfully co-ordinated and 

 explained by ionic hypotheses which are an extreme 

 form of molecular theory. Indeed, we have now 

 several methods by which the Individual atom 

 may be made manifest to our imperfect senses. 

 Crookes and Rutherford have shown that the 

 molecular bombardment of each individual a 

 particle from radium may be rendered visible by 

 the scintillation produced on a fluorescent screen 

 of zinc sulphide. 



