744 Mr. Charlp.^ E. S. Phillips [Feb. 11. 



be worked in un ordinary fnrnace. In glass making, the sand is 

 therefore heated with a salt of one or more of the alkaline group of 

 metals, preferably with sodium carbonate. k.i a moderate tempera- 

 ture sodium silicate is formed, and if this be subsequently heated in 

 the presence of either lead oxide 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 slab upon a hot brass plate. 



Many useful applications, besides providing 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 electricity may readily be obtained by 

 the simple device of allowing them to impinge upon a suitable sub- 

 stance. 



A 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. An 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. A 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 upon 

 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 

 Alter paper), as compared with a metal sheet, in producing the electri- 

 fication, appears to arise in the following way. A fine layer of dust 

 soon becomes firmly unbedded in the metallic surface, so that further 

 sand falling does not come into contact with the metal itself. On 

 the other hand it is probable that these particles cut through the 

 fibres of the paper and thus free themselves. I need hardly point 

 out that the filter papers used should not be specially dried. Pieces 

 which have been left about in a room for a few hours absorb 

 sufficient moisture to ensure the right degree of conductivity. 



The sign of the charge is always positive, in spite of the fact 

 that a rod of sihca rubbed upon the paper electrifies it negatively. 



