Ml I. HI (>F I'OSITIVKLV ANH NKCATIVKLY CHAKC5KI) IONS. 



293 



surface cil' tin- ln\\i-r plate. The slot \\;is closed exti-rn.-illy l>y ;i -trip of thin 

 .iluiiiiiiiiiin. cemented t< tin- glass \\ith -lii-llac. A focus Inilli. giving out Hontgen 

 rays. was !i\ed with the anticat hode oil a level with the upper surface of the lower 

 |>l:itc, tin- .inticatlio<li- In-ing placed with its plane almost hori/ontal. so that the 

 effective source of tin- rays which i-ntciv<l the slot was almost linear. A thick lend 

 screen was fixed, as shown in the figure, to pn-vent any rays reaching the interior of 

 the vessel alnive the level of the slot. Then- is thus oiilv a thin layer of air. close to 

 tin- surface of tin- li.\M-r plate, exjiosed to the direct action of the rays from the bull). 

 The lower plate was kept at zero potential, while the upper plate could l>e connected 

 either to the |w>sitive or negative terminal of a K-ittery, of which the other terminal 

 was earthed. The vessel could thus !>e charged at will with an excess of either 

 positive or negative ions. 



All the metal surfaces within the tube were covered with wet filter paper, to keep 

 the air saturated with water vapour, and to prevent nuclei lieing produced by the 

 action of the metal itself. 



A considerable Advantage is gained by having the plates horizontal, and the ionised 

 layer in contact with the lower plate, for any drops formed on the ions which are in a 

 minority (these In-ing confined to the lower part of the tube), have thus only a short 

 distance to fall, and if condensation takes place on these ions only, the drops will 

 be confined to the lower part of the vessel. 



In the experiment^ first performed with this apparatus the expansions were made 

 without previously cutting oft" the rays. 



The apparatus l>eing adjusted to give expansions somewhat exceeding the limit 

 V-i/Vi = 1'25, comparatively dense fogs were obtained when the upper plate was 

 maintained at a potential a few volts higher than the lower, so that negative ions 

 were present in excess : whereas, when the field was reversed (the positive ions being 

 now in excess) only a slight condensation could l>e observed, and this was mainly 

 confined to the region immediately over the lower plate, where a considerable number 

 of negative ions must have been present. With expansions as great as v.,/v t = 1*35 

 the api>earance of the fogs obtained was independent of the direction of the field, and 

 this continued to be the case up to the limit 1'38, at which dense fogs appear even in 

 the absence of ions. With the field in the direction which gives an excess of negative 

 ions, the density of the fogs which result from expansion is practically the same for 

 all values of t'g/t', between 1'28 and the alxwe-mentioned limit 1'38. When, on the 

 other hand, the upper plate is connected to the negative jxile of the battery, so that 

 the positive ions are in excess, the drops remain few till v.Jvi amounts to about P31, 

 when the number of the drojw logins to increase as the expansion is increased. With 

 t> 2 /V| = 1*33, we obtain, with the jxisitive ions, comparatively dense fogs, still, 

 however, considerably less dense than those obtained with negative ions. Finally, 

 altove I'-".:') the positive and negative fogs are indistinguishable. 



These results admit of only one interpretation ; condensation takes place on some 



