746 Mr. Charles E. S. Phillips [Feb. 11, 



instantly to earth, for in common with all powders it readily absorbs 

 moisture into the interstices between the grains. When making 

 electrical experiments with this material it is therefore essential to 

 have it well warmed. 



There is still much useful work to be done in studying- the 

 electrical conditions in the neighbourhood of wide stretches of warm 

 sand swept Ijy dry wind. Owing to lack of data, it is difficult to 

 form an opinion as to the part which tliis suljstance plays in the 

 remarkable electrical phenomena sometimes witnessed during a storm. 



I spoke of allowing sand to run down itself. Here is a cell 

 made by separating two glass plates, 14 inches square, by strips of 

 wood along the bottom and top edges. The sides are open. Through 

 a hole in the upper distance strip sand pours from a funnel and builds 

 itself into a beautifully symmetrical conic section. Presently the 

 base will so far widen that any further increase shoots the sand 

 off through the open ends of the cell. When this point is reached 

 the cone can no longer grow. A supply of white sand is then poured 

 in and seen to run down the sloping sides without carrying any of 

 the coloured particles with it. The base has spread out propor- 

 tionately as the cone increased in height, so that the angle which 

 the sides make with the horizontal shall be 35°. If the sand be 

 wet or damp this law no longer holds. The addition of sufficient 

 water materially diminishes the friction between tlie grains. 



It is often observed when walking along the sea-shore, upon sand 

 left wet by the receding tide, that for a moment the foot on touching 

 the ground is surrounded by a comparatively dry area. This appear- 

 ance is quickly followed, however, by one which indicates that the 

 sand has gathered moisture, for on lifting the foot — which has by 

 now probably sunk a little below the surface— the excess of water is 

 particularly noticeable. In order to explain this we must have re- 

 course to some ingenious experiments made a few years ago by Pro- 

 fessor Osborne Reynolds. He pointed out that a number of particles, 

 whether spheres or irregular grains, may lit together in such a way 

 that the size of the spaces enclosed by them is either a maximum or 

 minimum. Figs. 8 and 9 show a sectional view of a collection 

 of spheres, arranged in what Professor Reynolds calls abnormal 

 and normal piling respectively. It is evident that the spaces between 

 the spheres are far less in the second than in the first case. Now 

 here is an elastic bag tied upon one end of a glass tube. The arrange 

 ment is partly filled with sand and coloured water — the latter stand 

 ing 2 inches in the tube, so as to serve as an index. If the bag 

 is now tapped, all the particles in it become normally piled. We 

 have seen that any departure from this arrangement will enlarge the 

 spaces between them. It is no longer surprising to notice, therefore, 

 when the bag is pinched and the grains are thus made to ride up on 

 one another, that the liquid in the tube instead of rising, actually sinks. 



Returning to the effect observed upon the sea-shore, we see that 



