carried by the Ions produced by Bontgen Rays. 529 



radius the effect of the charge in promoting condensation 

 will more than counterbalance the effect of surface-tension in 

 preventing it. So that a charged ion will produce a very 

 small drop of water which may act as a nucleus. If each ion 

 acts as the nucleus for a drop, then if we know the size of the 

 drop and the mass of water deposited per unit volume, we 

 shall be able to determine the number of drops, and hence 

 the number of ions in unit volume of* the gas. One part of 

 the investigation is thus the determination of the size of the 

 drops: this gives us n; and as we know from the electrical 

 investigation ne, we have the means of determining e. 



The measurement of the size of the drops in the cloud gave 

 a great deal of trouble. Two methods were tried ; at first 

 I attempted to measure the size of the drops by an optical 

 method ; when a narrow beam of light from an arc lamp is 

 sent through the cloud, and the light after passing through 

 the cloud received on a screen, several coloured rings are 

 visible. If we assume that these rings arise entirely from 

 diffraction the size of the rings would enable us to deduce the 

 size of the drops. The method, however, failed in practice 

 from two causes. In the first place, in order to get the rays 

 bright enough to allow their diameter to be accurately 

 measured the fog must be dense, in order, however, to get a 

 dense cloud the number of ions produced by the rays must 

 be large ; when, however, the number of ions is large ex- 

 perience shows that they are not all brought down by the 

 first cloud formed by a sudden expansion, This is proved by 

 the fact that if after the first cloud has subsided, the rays 

 having been cut off immediately after the first expansion, 

 another expansion be made, a second cloud will be formed, 

 and though this is less dense than the first cloud it may 

 require two or three expansions to remove the effects of 

 previous exposure to the Rontgen rays. It is only when the 

 ions are so few that no cloud is produced by the second 

 expansion that we can feel any confidence that the number of 

 drops in the first cloud is equal to the number of ions formed 

 by the rays, and in this cnse the cloud is so thin that the 

 coloured rays are not bright enough to allow their diameters 

 to be accurately measured. Though this objection is fatal 

 there is yet another reason against using this method of 

 measuring the size of the drops, as observations made on the 

 dimensions of the various coloured rings seemed to indicate 

 that the rings are not produced entirely by diffraction, but 

 that they are influenced by the interference of rays which 

 have passed through the transparent drops with those which 

 have not done so, and that therefore we could not employ the 



