534 Prof. J. J. Thomson on the Charge of Electricity 



might get a cloud formed round one set of ions and not round 

 the other. The ions in the cloud would fall under gravity, 

 and thus we might have separation of positive and negative 

 ions and the production of an electric field, the work required 

 for the production of the field being done by gravity. 



To return, however, to the experiments under consideration. 

 The method employed for making the cloud and for measuring 

 the expansion is the same as that used by Mr. Wilson and 

 described by him in the ' Proceedings of the Cambridge Phi- 

 losophical Society,' vol. ix. p. 333. The gas which is exposed 

 to the rays is contained in the vessel A; this vessel communi- 

 cates by the tube B with the vertical tube C, the lower end 



To Earth 



To Electrometer 



of this tube is carefully ground so as to be in a plane perpen- 

 dicular to its axis, and is fastened down to the indiarubber 

 stopper D. Inside this tube there is an inverted thin- walled 

 test-tube, P, with the lip removed and the open end ground 

 so as to be in a plane perpendicular to the axis of the tube. 



