338 Effect of Rontg en s Bays on Cloudy Condensation. [Mar. 19, 



this arrangement. In all of them the potential varied under the 

 influence of the rays by an amount similar in direction and magnitude 

 to that previously observed. 



§ 13. I have observed that the activity of the vacuum bulb seems 

 to determine, to some extent, the potential difference observed on the 

 electrometer ; that is to say, if the rays are very weak and unsteady 

 (as judged by the fluorescence of the vacuum bulb) they do not make 

 the air sufficiently electrolytic to counterbalance the contact potential 

 difference between the surfaces of the plates. Thus, when the bulb 

 is not fluorescing brightly and steadily, one gets results which are 

 uncertain and perplexing. But these appear to give place in all cases 

 to more definite values whenever the rays are strong and steady. 



§ 14. The conclusions I have drawn from these experiments are 

 that (1) the influence of the rays on the zinc and tinfoil plates does 

 not cause any direct or sudden change in their contact potential, but 

 that (2) the air through which the rays pass is temporarily converted 

 into an electrolyte, and when in this condition forms a connexion 

 between the plates which has the same properties as a drop of acidu- 

 lated water, namely, it rapidly reduces the potential between the 

 opposing surfaces of the plates to zero, and may even reverse it to a 

 small extent. 



It is interesting to note that this electrolytic property was found by 

 Lord Kelvin ('Electrostatics and Magnetism,' Art. XXIII, §§ 412— 

 414) to be possessed by the fumes from a burning spirit lamp. In 

 both cases its cause is probably the same. It is, no doubt, due to a 

 want of electrical equilibrium among, and a partial dissociation of, 

 the molecules of the gas. 



VI. " The Effect of Rontgen's Rays on Cloudy Condensation." 

 By C. T. R. Wilson, B.Sc. (Vict.), B.A. (Cantab.), Clerk- 

 Maxwell Student. Communicated by Professor J. J. 

 Thomson, F.R.S. Received March 3, 1896. 



In a paper on " The Formation of Cloud in the Absence of Dust," 

 read before the Cambridge Philosophical Society, May 13th, 1895, I 

 showed that cloudy condensation takes place in the absence of dust 

 when saturated air suffers sudden expansion exceeding a certain 

 critical amount. 



I find that air exposed to the action of Rontgen's rays requires to 

 be expanded just as much as ordinary air in order that condensation 

 may take place, but these rays have the effect of greatly increasing 

 the number of drops formed when the expansion is beyond that neces- 

 sary to produce jondensation. 



Under ordinary conditions, when the expansion exceeds the critical 



