858 Professor Osborne Beynolds [Feb. 12, 



the pressure of tlie air in the pores is paradoxical, and shows the 

 anti-sponginess of the granular material ; had there been a sponge in 

 the bag, the pressure of the air would have increased with the 

 squeezing. 



This experiment has been mainly introduced to prevent a possible 

 impression that the fluid filling the interstices has anything to do with 

 the dilation besides measui'ing it. 



Water affords a more definite measure of volume than air. 



Taking a small indiarubber bottle with a glass neck full of shot 

 and water, so that the water stands well into the neck. If instead of 

 shot the bag were full of water or had anything of the nature of a 

 sponge in it, when the bag was squeezed the water would be forced 

 up the neck. With the shot the opposite result is obtained; as I 

 squeeze the bag, the water decidedly shrinks in the neck. 



This experiment, which you see is on a very small scale, was not 

 designed to show to an audience ; it was the original experiment 

 which was made for my own satisfaction, when the idea of dilatancy 

 first presented itself. The result but for the knowledge of dilatancy 

 would appear paradoxical, not to say magical. When we squeeze a 

 sponge between two planes, water is squeezed out ; when we squeeze 

 sand, shot, or granular material, water is drawn in. 



Taking a larger apparatus, a bag which holds six pints of sand, 

 the interstices of which are full of water without any air — the glass 

 neck being graduated so as to measure the water drawn in. On 

 squeezing the bag with a large pair of pincers, a pint of water is 

 drawn from the neck into the bag. This is the maximum dilation ; 

 the grains of sand are now in the most open order into which they 

 can be brought by this squeezing ; further squeezing causes them to 

 take closer order, the interstices diminish, and the water runs out into 

 the vessel, and for still further squeezing is drawn back again, show- 

 ing that as the change of form continues, the medium passes through 

 maximum and minimum dilations. 



This experiment may be repeated with granules of any size or 

 shape, provided only they are hard, and shows the universality of 

 dilatancy. 



Although not more definite, perhaps more striking evidence of 

 dilatancy is afforded by the means which the non-expansibility of 

 water affords of limiting the volume of the bag. An impervious bag 

 full of sand and water without air cannot have its contents enlarged 

 without creating a vacuum inside it — the interstices of the sand are 

 therefore strictly limited to the volume of the water inside it, unless 

 forces are brought to bear sufficient to overcome the pressure of the 

 atmosphere and create a vacuum. Since then, owing to this property 

 of dilatancy, the shape of a granular mass at its greatest density cannot 

 change without enlarging the interstices, preventing this enlargement 

 by closing the bag we prevent change of shape. 



Taking the same bag, the sand being at its closest order — 

 closing the neck so that it cannot draw more water. A severe pinch 



