PHYSICS. 583 



form is imposaible. Thus a sack of corn is flexible on end, but becomes 

 hard when placed on its side, and its shape will not alter. But if the 

 sack be made of rubber, since the boundary of the granular mass is 

 now extensible, it remains perfectly flexible in all positions. However, 

 if it be possible with an extensible envelope to impose a maximum vol- 

 ume upon the contents, efiects similar to those obtained with the inex- 

 tensible boundary may be expected. This can be done, for example, 

 by placing No. C shot in a rubber bag and adding a certain amount 

 of water. If the quantity of water be such that the spaces between 

 the granules when in close arrangement are all filled by it, while 

 with a wide arrangement the amount is not enough, a point will be 

 reached in passing from the first to the second arrangement such that 

 any further change of shape and consequently of volume would produce 

 a vacuum. When thisstageisreachedthewholemass becomes perfectly 

 hard. When the foot presses upon wet sand, that portion of it imme- 

 diately surrounding the foot becomes momentarily dry. The sand being 

 completely filled with water, the pressure of the foot causes dilatation, 

 and so more water is required. This is drawn in from the surrounding 

 sand leaving it dry until a suilBcient supply has been drawn up from 

 below. On raising the foot the sand contracts again and the excess of 

 water escapes again wetting the sand under and around the foot. The 

 author conceived that the property of dilatancy placed a hitherto un- 

 known mechanical contrivance at the command of those who would ex- 

 plain the fundamental arrangement of the universe ; and he proceeded 

 to explain how bodies in such a medium would, in virtue of the dilation 

 caused in the medium, attract each other at a distance with a force de- 

 l)ending on the distance which might well correspond with gravitation. 

 Further, owing to the existence of a region close to the body in which 

 the density varies several times from maximum to minimum, the mutual 

 force might undergo a change from attraction to repulsion, and this 

 more than once, as the bodies approach a condition which seems to 

 account for cohesion and observed molecular force far better than any 

 previous hyi)othesis. {Phil. Mag., December, 1885, V, xx, 469 ; Nature, 

 October, 1885, xxxii, 535.) 



Von Helmholtz has suggested an improved method of measuring the 

 modulus of elasticity of solids. Ordinarily the bar is firmly supported at 

 its ends and the flexure produced by loading it in the middle is measured 

 with the cathetometer. Under these circumstances, however, a consid- 

 erable error is introduced by the compression at the points of support. 

 To eliminate this Von Helmholtz uses two perpendicular mirrors at the 

 ends of the bar, their reflecting surfaces being directed inwards. A 

 scale is placed opposite one end of the bar and a reading telescope op- 

 posite the other. The image of the scale is seen in the one mirror as 

 reflected in the opposite one. As the bar is loaded and becomes flexed 

 the image is displaced to an amount proportional to the angular changes 

 of the mirrors. The amount of flexure for diflerent loads is a simple 



