256 



NATURE 



[August 25, 1910 



however, that the formation of a sand dune is very fre- 

 quently due to wind eddies. The second photograph was, 

 in fact, talcen by him in Egypt, and depicts the steady, 

 irresistible march of millions of tons of sand, encroaching 

 upon and slowly burying casuarina trees (Fig. 2). 



To come nearer home, the seriousness of problems arising 

 out of this state of things may be illustrated by two photo- 

 graphs obtained recently at Southport, in Lancashire. In 

 the first one (Fig. 3), the bacli garden of a newly built 

 house is nearly buried beneath the enormous hill, which 

 will probably soon cover the whole property. The second 

 (Fig. 4) shows that the familiar appearance of a sandy 

 beach at low vifater, with regular lines of ripples, may be 



produced by the direct action of the wind, and, incidentally, 

 the utter futility of constructing an esplanade in such a 

 neighbourhood. All these phenomena depend, in some 

 measure, upon the size, weight, and shape of the sand 

 grains themselves. 



Silica, a substance which occurs in numerous impure 

 forms, and constitutes a large portion of the rock masses 

 known to geologists, is also to be found in a pure state 

 as crystalline quartz. Here is an actual specimen about 

 18 inches long, which, together with the beautiful group 

 of quartz crystals by its side, known as amethysts (and 

 tinted, probably, by a trace of organic matter), are the 

 property of this institution. Sand, therefore, being the 

 result of rock disintegration, assisted by the grinding action 

 NO. 2130, VOL. 84] 



due to the motion of wind or water, varies in composition 



in different localities. 



The next slides are photomicrographs taken with a low- 

 power objective. They represent some grains of sand 

 found at Charlton and the Isle of Eigg respectively (Figs. 

 5 and 6). The former are seen to consist of minute silica 

 particles of very irregular form, whereas the larger grains 

 of the Eigg sand are remarkable for their smoothness. 

 It is owing to this fact that the latter possess a peculiar 

 property, to be referred to later. 



Owing to the Sahara Desert having once formed the 

 bed of a vast sea, it is, of course, found to be rich in 

 nuu'inf deposit. 



The damage which sand is capable of 

 doing has been already referred to. It 

 must not be forgotten, however, that its 

 utility in the arts and crafts is of the 

 utmost importance. The Egyptians are 

 reputed to have been the first to find a 

 wide use for it. They were probably the 

 earliest glass-workers in the world. By 

 the time glass-making was begun in 

 England, viz. about 161 1, the Romans and 

 Venetians had so far mastered the art of 

 blending sand with other substances that 

 almost all the technical difficulties had 

 already been overcome. 



Now the melting point of silica being 

 about 3000° C, it cannot be worked in an 

 ordinary furnace. In glass-making the 

 sand is therefore heated with a salt of 

 one or more of the alkaline group of 

 iiietals, preferably with sodium carbonate. 

 .'\t a moderate temperature sodium silicate 

 is formed, and if this be subsequently 

 heated in the presence of either lead o.xide 

 or borax, the melting point of the mass 

 is still further reduced. Here is a white- 

 hot crucible containing sand so treated and 

 melted. You see the glass pours out like 

 treacle, and sets rapidly into a transparent 

 >lab upon a hot brass plate. 



Many useful applications, besides pro- 

 viding us with windows and glass-ware, 

 have been found for sand, such as the 

 decorating of hard surfaces by means of 

 an impinging stream of its particles, 

 scouring and cleaning, preventing slip on 

 the roads, and so on. By no means the 

 least important of these is its employment 

 in war as a protection against bullets; a 

 thickness of 20 inches of dry sand is 

 proof against the modern rifle. 



Now a mass of sand grains moving 

 down a slope, by a motion consisting of 

 rolling and sliding, meets with great 

 opposition, due to friction. The grains 

 thus come into close contact with the 

 surface, and a considerable charge of elec- 

 tricity may readily be obtained by the 

 simple device of allowing them to impinge 

 upon a suitable substance. 



\ stream of sand flowing from the base 

 of this reservoir B (Fig. 7) strikes upon 

 an oblique sheet of tin T, which is 

 attached to an insulating pillar N. .^n 

 electrostatic voltmeter connected with 

 the metal plate serves to measure 

 the electrical potential. You see that in a moment the 

 tin becomes charged to 3000 volts. The needle, however, 

 soon falls back. Something has changed. The plate has, 

 in fact, become dulled and pitted where the sand struck it. 

 .\ fresh part reproduces the high potential. Filter paper is 

 far more serviceable, and so is a wooden surface. One 

 may rapidly obtain a potential of 6000 volts if the sand 

 fall jjpon paper or wood, and this can be maintained for 

 a considerable time. If the reading of the voltmeter 

 diminishes, a fresh portion of the surface offered to the 

 sand stream immediately brings it to its original value 

 as before. The greater efficiency of paper (preferably 

 filter paper) as compared with a metal sheet In producing 

 the electrification, appears to arise in the following way. 



