482 



Prof. W. A. Douglas Hudge on the 



of dust composed of various materials, by artificial means. 

 As lias been shown *, the electrification developed during 

 dust storms in South Africa is invariably negative, neutral- 

 izing and reversing the positive charge usually present in 

 the air. In England, in places where the matter was tested, 

 the clouds of dost raised by motor-cars had the effect of 

 increasing the positive charge. In South Africa the dust is 

 usually of a siliceous character, whether it comes from the 

 sea-shore, inland wastes, cultivated soils, or the mine-heaps 

 in the neighbourhood of Johannesburg. In England " road 

 dust " is usually calcareous. It is quite easy to show the 

 production of a charge of electricity by raising a cloud of 

 •dust. If a small quantity of flour is placed upon the plate 

 of an electroscope and then suddenly blown away, the 

 electroscope will indicate a charge whose magnitude will 

 depend to some extent on the fineness of the flour. The 

 •experiment can be performed in a room, but the effect is 

 very much greater if it is done out in the open air. By 

 raising a cloud of flour-dust in any way and catching the 

 ;particles in a hollow insulated vessel, it is always possible 

 ito «how the presence of a charge of electricity. 



The apparatus used for systematic investigation consisted 

 of a small cylinder of fine wire-gauze 10 cm. in length and 

 2 cm. in diameter, which was attached to a hollow brass 

 cylinder of the same diameter. A nozzle projected a short 



Fig. 1. 

 Wire gauze Brass tube 





JL 











distance into the brass cylinder, and if some powder was 

 arranged in front of the nozzle a sharp puff of air would 

 blow it into a cloud, and the particles would be. carried 

 through the meshes of the wire gauze and give up a portion 



* Phil. Mag. June 1912; Proc. Roy. Soc. South Africa, 1912; South 

 African Journ. Sci. 1912. 



