Electrification associated with Dust- Clouds. 483 



o£ their charges to it (fig. 1). The cylinder was insulated 

 by an ebonite rod and connected to a sensitive gold-leaf 

 electroscope o£ the pattern designed by C. T. R. Wilson. 

 An ordinary instrument suffices in most cases, as the charges 

 developed are quite strong. The air current was usually 

 obtained from a Fletcher bellows, but a sharp puff from 

 the mouth was quite efficacious. When fine sand or dry 

 earth was used a strong positive charge was indicated, 

 as would have been expected from what was shown in a 

 previous paper * ; but on using other material, the charge 

 was positive or negative, according to the nature of the 

 material. 



Almost the only references to the electrification of dusts 

 are those relating to the production of Lichtenberg's figures, 

 by shaking a mixture of red lead and sulphur in a muslin 

 bag, and allowing the powders to fall upon an irregularly 

 electrified plate composed of some non-conducting material. 

 A separation of the red lead and sulphur particles is effected 

 because of the opposite charges acquired by these materials, 

 the sulphur particles being stated to be negatively electrified, 

 whilst those of the red lead are positive. Now, powdered 

 sulphur when blown into a cloud in the apparatus acquires 

 a positive charge, and red lead a negative one. This fact 

 seemed sufficient to warrant an investigation into the kind 

 of electrification accompanying the raising of clouds of dust 

 from different materials. 



A great many experiments were made with those materials 

 usually found in the chemical laboratory, and very shortly it 

 was seen that the electrification developed appeared to follow 

 some general laws ; and although there are many apparent 

 exceptions — probably due to the use of materials whose 

 purity is doubtful — the laws given below are fairly well 

 established by the experiments. The materials used were, 

 taken from the stock-bottles in the laboratory and no attempt 

 was made to purify them. The experiments were conducted 

 out of doors, and the results are tabulated below. 



Law I. Non-metallic elements give positively charged clouds 

 lohen the finely divided material is blown into a cloud by 

 a current of air. 



Of course, only solid elements can be dealt with in this 

 manner. 



* Phil. Mag. June 1912. 

 2 L 2 



