MATTER 263 



but which is reached in conception by carrying on 

 indefinitely a process (or a classification of compressible 

 bodies) starting in perception. 



Turning from this agreement to the divergence between 

 water and jelly, we remark that if a lath of wood or even 

 a knife-blade be pressed downwards on a jelly it requires 

 considerable effort to shear or separate the jelly into two 

 parts ; on the other hand, the water is separated by the 

 lath without any sensible resistance. Now the change of 

 shape we are in this case concerned with is of the nature 

 of a slide (p. 204), and we say that the water offers little 

 and the jelly considerable resistance to sliding strain. 

 Here, again, the question of the amount of resistance is 

 relative. So far as our perceptual experience goes, all 

 fluids offer some, however small, resistance to the sliding 

 of their parts over each other. The fluid which offers 

 absolute resistance to compression and no resistance at 

 all to slide of its parts — or the parts of which slip over 

 each other without anything of the nature of frictional 

 action — is only a conceptual limit. Such a fluid is 

 termed 2. perfect fltiid. On the other hand, by proceeding 

 to the opposite limit in the case of an incompressible 

 jelly, that is, by supposing it to resist absolutely change 

 of shape by sliding, we should obtain a body incapable of 

 changing its form by either compression or slide, and thus 

 reach that conceptual limit, the rigid body. If we suppose 

 absolute resistance to compression and partial resistance 

 to slide, we have in conception a medium which might 

 perhaps be described as di. perfect jelly. 



Returning now to our ether, we note that physicists 

 conceive it incompressible, but that for some purposes 

 they appear to treat it as a perfect fluid, for other purposes 

 as a perfect jelly} This might at first sight appear a 

 contradiction or conflict of conceptions, and it does 

 undoubtedly involve difficulties which physicists are at 

 present far from having thoroughly mastered. If we con- 

 sider the ether as purely conceptual, then, in order to 

 describe different phases of phenomena, we are certainly at 



' For further purposes again scarcely as either. 



