1886.] on Experiments showing Dilatancij. 361 



interstices ancl gives the sand colierence. The island grows vertically, 

 very fast, and assumes the form of a column, sometimes with branches 

 like a tree or a fern, some inches or even a foot high. The strength 

 of these consists in the surface tension of the water preventing air 

 from being drawn in to enlarge the interstices, which therefore 

 cannot change shape ; it is therefore another evidence of dilatancy. 



By substituting an impervious envelope for the surface of water, 

 firmness of sand saturated with water may be rendered very striking. 



Thin indiarubber balloons, which may be easily expanded with the 

 mouth, afford an almost transparent envelope. 



Taking one with about six pints of sand and water closed without 

 air, there being more water than will fill the interstices at the densest, 

 but not enough to allow them to enlarge to the full extent. When 

 standing on the table, the elasticity of the envelope given is a rounded 

 shape. The sand has settled down to the bottom, and the excess of 

 water appears above the sand, the surface of which is free. The bag 

 may be squeezed and its shape altered, apparently as though it had 

 no firmness, but this is only so long as the surface is free. But taking 

 it between two vertical plates and squeezing, at first it submits, 

 apparently without resistance, when all at once it comes to a dead 

 stop. Turning it on to its side, a 56-lb. weight produces no further 

 alteration of shape ; but on removing the weight, the bag at once 

 returns to its almost rounded shape. 



Putting the bag now between two vertical plates, and slightly 

 shaking while squeezing, so as to keep the sand at its densest, while 

 it still has a free surface, it can be pressed out until it is a 

 broad flat plate. It is still soft as long as it is squeezed, but the 

 moment the pressure is removed, the elasticity of the bag tends to 

 draw it back to its rounded form, changing its shape, enlarging the 

 interstices, and absorbing the excess of water ; this is soon gone, and 

 the bag remains a flat cake with peculiar properties. To j)ressures 

 on its sides it at once yields, such pressures having nothing to over- 

 come but the elasticity of the bag, for change of shape in that 

 direction causes the sand to contract. To radial pressures on its rim, 

 however, it is perfectly rigid, as such pressures tend further to dilate 

 the sand ; when placed on its edge, it bears one cwt. without 

 flinching. 



If, however, while supporting the weight it is pressed sufficiently 

 on the sides, all strength vanishes, and it is again a rounded bag of 

 loose sand and water. 



By shaking the bag into a mould, it can be made to take any 

 shape ; then, by drawing off the excess of water and closing the bag, 

 the sand becomes perfectly rigid, and will not change its shape without 

 the envelope be torn ; no amount of shaking will effect a change. In 

 this way bricks can be made of sand or fine shot full of water 

 and the thinnest indiarubber envelope, which will stand as much 

 pressure as ordinary bricks without change of shape ; also permanent 

 casts of figures may be taken. 



