460 Mr. C. T. B. Wilson [Feb. 19, 



continuallj replaced by others mauiifactured within the apparatus 

 itself. 



To produce the necessary supersaturation to cause condensation 

 in the form of drops in dust-free air, the air must be allowed to 

 expand suddenly till the final volume is 1*25 times the initial 

 volume. The condensation is rain-like in form, and moreover the 

 number of drops remains small although the expansion considerably 

 exceeds this lower limit. Expansions exceeding the limit, Yo/^i 

 = 1 • 38, however, give fogs, which increase rapidly in density, i.e. 

 in the number of the drops, as the expansion is increased beyond this 

 second limit. The exj)ansions required for the rain-like and cloud- 

 like condensations correspond to a fourfold and eightfold super- 

 saturation respectively. 



A further experiment will throw light on the nature of the nuclei 

 associated with the rain-like condensation. Let us expose the moist 

 air to the action of X-rays before causing it to expand. First let us 

 try an expansion very slightly less than that required to give the 

 rain-like condensation without the rays. You observe, no drops are 

 formed. Now let the expansion be slightly greater than the critical 

 value, 1*25. A fog is seen on expansion. Thus the X-rays produce 

 in the air immense numbers of nuclei having the same properties, so 

 far as their power of assisting condensation goes, as the compara- 

 tively few nuclei which the rain-like condensation makes visible. 

 Now, a gas exposed to X-rays conducts electricity, and the otherwise 

 complicated phenomena of this conduction are all reduced to com- 

 parative simplicity by the theory that under the action of the rays 

 equal numbers of freely moving positively and negatively electrified 

 bodies (the ions) are produced from the originally neutral gas. It is 

 at once suggested that the condensation nuclei produced by X-rays 

 are simply these ions. 



Let us now impart conducting power to the gas by exposing it to 

 the action of the radiation from radium. Again we have the same 

 result, no drops produced if the expansion be less than 1*25, fog if 

 the expansion exceed this limit. 



If we substitute for the glass shade, which has thus far formed 

 the cloud-chamber, a glass cylinder with a horizontal metal top, we 

 have the means of testing w^hether the condensation nuclei produced 

 by Eontgen or radium rays are really electrically charged, whether 

 in fact it is the ions themselves which act as condensation nuclei or 

 other particles produced by the rays. If, for example, the roof of 

 the cloud chamber be kept positively charged and the floor negatively, 

 the negatively charged ions will travel upwards and the positively 

 charged ones downwards. In the absence of an electric field the 

 positive and negative ions produced by the action of the rays will go 

 on increasing in number until as many are neutralised by recom- 

 bination with ions of the opposite kind, or by coming in contact with 

 the walls of the vessel, in each second, as are set free in that time 

 by the rays. If the rays be cut off", the removal of ions by recom- 



