Physics and Chemistry. 217 



is entirely different. So long as the grains are held in mutual 

 equilibrium by stresses transmitted through the mass, every 

 change of relative position of the grains is attended by a conse- 

 quent change of volume ; and if in any way the volume is fixed, 

 then all change of shape is prevented. Thus a group of equal 

 spheres being so arranged that if the external spheres are fixed, 

 the internal ones cannot move, any distortion of the boundaries 

 will cause an alteration in the mean density depending on the dis- 

 tortion and the arrangement of the spheres. If a canvas bag con- 

 tain hard grains or balls, so long as the bag is not nearly full it 

 will change its shape as it is moved about ; but when it is approx- 

 imately full a small change of shape causes it to become perfectly 

 hard. If instead of a canvas bag, an extremely flexible bag of 

 india rubber be filled with heavy spheres (No. 6 shot), the envel- 

 ope imposes no sensible restraint on their distortion ; so that when 

 standing on the table it takes nearly the form of a heap of shot. 

 But if the interstices between the shot be filled with water so that 

 the bag is quite full of water and shot, no bubble of air being con- 

 tained in it, and the mouth be carefully closed it will be found that 

 the bag has become absolutely rigid in whatever form it happened 

 to be when closed. Since neither the envelope nor the water im- 

 poses any distortional constraint on the shot, what it is which con- 

 verts the heap of loose shot into an absolutely rigid body ? Clearly 

 the limit which is imposed on the volume by the pressure of the at- 

 mosphere. So long as the arrangement of the shot is such that 

 there is enough water to fill the interstices, the shot are free ; 

 but any arrangement which requires more room is absolutely 

 prevented by the pressure of the atmosphere. The existence of 

 dilatancy in sand explains the well marked phenomenon observed 

 in walking on a wet beach. As the foot falls on the sand, it 

 whitens or appears momentarily to dry around the foot. When 

 this happens the sand is full of water, the surface of which is kept 

 up to that of the sand by capillary attraction. The pressure of the 

 foot causes dilation of the sand and more water is required which 

 has to be obtained either by depressing the level of the surface 

 against the capillary attraction or by drawing water through the 

 interstices of the surrounding sand. The latter requires time to 

 accomplish ; so that for a time the capillary forces are overcome, 

 the surface of the water is lowered below that of the sand, leaving 

 the latter dryer until a sufficient supply has been obtained from 

 below. On raising the foot it is generally seen that the sand 

 under the foot and immediately around it is wet ; because the dis- 

 torting forces being removed the sand again contracts and the 

 excess of water finds momentary relief at the surface. In the 

 opinion of the author the recognition of this principle of dilatancy 

 places a hitherto unrecognised mechanical contrivance at the com- 

 mand of those who would explain the fundamental arrangement 

 of the universe, and one which seems to promise great things. 

 For example, hitherto no medium has ever been suggested which 

 would cause a statical force of attraction between two bodies at a 



