GASES BY THE ACTION OF EONTGEN BAYS AND OTHEE AGENTS. 
433 
of drops in the compartment next the metal v/as generally greater than on the other 
side of the partition. 
With amalgamated zinc comparatively dense fogs are obtained with such 
expansions ; polished zinc and lead also show the effect well; polished copper and 
tin produce no appreciable effect. A plate of zinc amalgamated on one side and 
merely polished on the other shows a great difference in the density of the fog on the 
two sides, the fog next the amalgamated metal l)eing much the denser. If the 
partition consists of lead with an old surface on one side and a freshly scraped surface 
on the other, mamy more drops appear in the half of the tube next the fresh surface. 
With a zinc copper partition also, the drops are much more numerous on the zinc side. 
With amalgamated zinc a few drops may he produced even when VgA’i is rather 
less than 1'25 ; they have been observed with a pressure fall of only 13 centims. 
The effect of the metals is much more marked when the expansion is considerably 
greater {ih/vx = 1'30 or more) ; in many cases, indeed, the effect of the metal was 
inappreciable with smaller expansions. When v^jv^ exceeds 1’38 no difference can be 
detected iDetween the comparatively dense fogs which then occupy both sides of the 
tube. 
There can be little doubt that the effect here described is due to the same cause as 
the influence which these metals have on a photographic plate, studied by Russell'^'' 
and others. As far as these experiments go they tend to show that the order in 
which the metals must be arranged to indicate their relative activity in producing 
nuclei is the same as their order when arranged according to their photograpliic 
activity ; amalgamated zinc giving the most effect; tin and copper little effect, if any ; 
polished zinc and lead being intermediate in activity. 
The experiments described above were all performed with air in the expansion 
apparatus. In experiments with hydrogen in the apparatus the metals (zinc, 
amalgamated zinc, and lead) showed only a very slight effect. This, however, does 
not prove conclusively that more nuclei are produced when the metal is in contact 
with air than with hydrogen ; the difference may he due to the more rapid removal 
of the nuclei in hydrogen by diffusion to the walls of the vessel. 
I have not been able to obtain amy effect from metals outside the apparatus, even 
through celluloid, which Russell found to he })enetrated by the photographic action. 
§ 8. Nuclei produced by the Action of Ultra-violet Light on a 
Negatively Electrified Zinc Plate. 
Lenard and Wolff (/oc. cit.) were alile to show that the condensation of a steam 
jet becomes dense in the neighbourhood of a negatively electrified zinc plate when 
this is exposed to the action of ultra-violet light. 
The delicacy of the expansion method makes it a matter of some difficulty to 
*Eussell, ‘Eoy. Soc. Proc.,’vol. 61, p. 424,1897; vol. 63, p. 102,1898. Colson, ‘C. E.,’vo1. 123, p. 49,1896. 
YOL. CXCII. - A, 3 K 
