Electrification associated ivith Dust- Clouds. 491 



however, to stop most of the dust and to deal mainly \\ith 

 the charge carried by the air. 



A small cylinder was constructed or! several layers of the 

 finest wire gauze obtainable, and this was covered over with 

 several layers of " chiffon/'' so that only the finest particles 

 of dust could pass, but the air could do so without much 

 hindrance. Using this cylinder, it was found possible to 

 stop most of the dust, so that the charge acquired by the 

 outer cylinder was mainly that due to the air. Some of the 

 dust did get through, but probably most of the charge 

 it possessed was removed by keeping the inner cylinder 

 " earthed." Table X. gives the results obtained in some 

 typical cases. The inner cylinder was insulated and earthed 

 alternately. When insulated, the charge indicated by the 

 outer cylinder was that due to the dust particles which suc- 

 ceeded in getting through, and also to the inductive effect 

 of the inner cylinder, which had caught most of the charge 

 upon the dust. On earthing the inner cylinder, the charge 

 upon the dust would be removed, but sufficient would be 

 carried by the air to give a definite charge to the outer 

 cylinder. 



Table X. 



Substance. 



Inner cylinder. 



Charge upon 

 outer cylinder. 





Insulated. 

 Earthed. 



+ 



Sulphur 



Insulated. 

 Earthed. 



+ 







Insulated. 

 Earthed. 



+ 



Litharge 



Insulated. 

 Earthed. 



+ 





Boracic acid 



Insulated. 

 Earthed. 



I 

 + 





These examples show very conclusively that charges of 

 opposite sign are carried by dust and by air. 



Another form of apparatus was also devised, by which the 

 dust particles could be separated from the air by allowing them 

 to settle upon an insulated plate and then determining the 



