202 J. A. POLLOCK. 



paper just mentioned. In order to get small expansion 

 ratios the glass cylinder in which the clouds were formed 

 had a volume of 8647 cubic centimetres and the piston was 

 reduced to a pipe 2*3 centimetres in diameter. The cylinder 

 was divided vertically by an earthed metal plate on one 

 side of which and parallel to it was an insulated one, so 

 that an electric field could be established in one half of the 

 vessel. With this apparatus dust motes could usually be 

 seen in the beams of light which were used to illuminate 

 the two halves of the cylinder, and the first condensations 

 visible on expansion occurred for pressure differences repre- 

 sented by about 1*5 centimetres of water. Comparative 

 observations were made with the field off, and on for a 

 sufficient time before expansion for all the large ions 

 originally present to be attracted to the plates, but we have, 

 so far, been unable to find any influence of the field on the 

 limiting expansion ratio required for noticeable conden- 

 sation. 



The attachment of small ions to dust particles can, 

 however, be readily shown, for if the air in the expansion 

 chamber is exposed to Rontgen rays while the field is on, 

 the clouds on the field side of the vessel get less and less 

 dense the longer the time of application of the rays before 

 expansion. Under the conditions of the experiments, if the 

 rays and field were on for about forty minutes before 

 expansions giving pressure differences of two to three 

 centimetres of water, only a few drops were formed in the 

 field side of the dividing plate while dense fogs appeared 

 in the other. Obviously sufficient small ions were pro- 

 duced in the interval before expansion to electrify by their 

 attachment nearly the whole of the dust particles present, 

 those in the field side being then carried to the plates by 

 the electrical forces and the space on that side of the 

 dividing plate thus cleared of condensation nuclei. This 



