1886.] o?» Experiments showing Dilatancy. 363 



reason for calling your attention to them was partly an experiment, 

 which, although not the most striking, is the most advanced experi- 

 ment in the direction of dilatancy. 



The apparatus is that represented in the diagram ; the medium is 

 contained in the large elastic bag ; in the middle of this bag is a 

 small hollow elastic ball, which can be expanded by water forced in 

 through a tube passing through the medium and outside ball ; the 

 quantity of water which passes in is measured by a mercury gauge, 

 the water being forced in by the pressure of the mercury. The 

 medium between the two balls is sand and water, and is connected 

 with a gauge, the water drawn from which measures the dilation. 



The full pressure of 30 inches is on the interior ball, but produces 

 no expansion, because the medium outside cannot dilate as the supply 

 of water is now cut off; opening the tap to admit water to the outer 

 ball, it at once draws water. It has now drawn 3 oz. ; in the mean- 

 time the mercury has fallen, showing that an ounce and a half was 

 admitted to the interior ball, the expansion of which drew the water 

 into the outer envelope. This experiment is not striking, but it is 

 definite, and enables us to measure the dilation consequent on a given 

 distortion. 



It is impossible for me to go further into this exjDlanation, so I 

 will merely state that the ability of the grains of a medium to slide 

 over a smooth surface has been experimentally shown to produce 

 phenomena closely resembling the conduction of electricity, to com- 

 plete which it is only necessary to construct the medium of two 

 different sorts of grains, different in size or different in shape, the 

 separation of which would afford the two electricities and be a 

 simple way out of the difficulty hitherto found in explaining the 

 non-exhaustibility of the electricity in a body. Hitherto the 

 two electric fluids have been supposed to reside together in 

 the matter of the machine, which, however much has been with- 

 drawn, has never shown signs of exhaustion. In the dilatant hypo- 

 thesis these electricities are the two constituents of the a3ther which 

 the machine separates, and it is worth noticing that the ordinary 

 electrical machine resembles in all essential particulars the machines 

 used by seedsmen for separating two kinds of seed, trefoil and rye- 

 grass, which grow together : as long as there is a supply of the mixture, 

 the machine is never exhausted. 



This dilatant hypothesis of aether is very promising, although it 

 cannot be pat forward as proved until it has been worked out in 

 detail, which will take long. In the meantime it is put forward 

 mainly to excite interest in the property of dilatancy to the discovery 

 of which it has led. This property, now that it has once been recog- 

 nised, is quite independent of any hypothesis, and offers a new field 

 for philosophical and mathematical research quite independent of the 

 sether. 



[0. E.] 



