1886.] on Experiments sJiowing Dilatancy. 357 



hitherto been the most commonplace and least interesting arrange- 

 ments. That an important geometrical and mechanical property of 

 a material system should have been hidden for thousands of years, 

 even in sand and corn, is such a striking thought that it required no 

 little faith in mechanical principles to undertake the search for it, 

 and although finding nothing but what was strictly in accordance 

 with the conclusions previously arrived at, the evidence obtained of 

 this long-hidden property was as much a matter of visual surprise to 

 the lecturer as it can be to any of the audience. 



To render the dilatancy of a granular mass evident, it was neces- 

 sary to accomplish two things : (1) the outside grains must be con- 

 trolled so that they could not rearrange, and this without preventing 

 change of shape and bulk of the mass ; (2) the changes of bulk or 

 volume of the mass, or of the interstices between the grains, must be 

 rendered evident by some method of measurement which did not de- 

 pend on the shape of the mass. 



A very simple means — a thin indiaruhher envelope or boundary — 

 answered both these purposes to perfection. The thin indiarubber 

 closed over the outside grains sufficiently to prevent their change of 

 position, and the impervious character of the bag allowed of a con- 

 tinuous measure of the volume of the contents, by measuring the 

 quantity of air or water necessary to fill the interstices. 



Taking an indiarubber bag which will hold six pints of water, 

 without stretching, and having only a small tubular aperture, getting 

 it quite dry, and putting into it six pints of dry sea sand, such as will 

 run in an hour-glass, sharp river sand, dry corn, shot, or glass 

 marbles, it presents no very striking appearance, but all the same 

 when filled with any of these materials, it cannot have its form 

 changed as by squeezing between two boards without changing its 

 volume. These changes of volume are not sufficient to be noticeable 

 while the squeezing is going on, but they may be rendered apparent. 

 It is sufiicient to do this with the bag full of clean dry Calais sand, 

 such as is used in an hour-glass. 



The tube from the bag is connected with a mercurial pressure- 

 gauge, so that the bag is closed by the mercury. 



The actual volume occupied by the quartz grains is four and a 

 half pints. The remaining space, one and a half pints, is occupied by 

 the interstices between the grains in their closest order ; these inter- 

 stices are full of air, so that three-quarters of the bag are occupied by 

 quartz, and one-quarter by air. Since the bag is closed and no more 

 air can get in if interstices are increased from one pint and a half to 

 two pints, the air must expand, and its pressure will fall from that of 

 the atmosphere to three-quarters of an atmosphere. As soon as 

 squeezing begins, the mercury rises on the side connected with the 

 bag, and steadily rises as the bag flattens until it has risen seven inches, 

 showing that the bag has increased in capacity by half a pint or one- 

 twelfth of its initial capacity. 



That by squeezing a porous mass like sand we should diminish 



