GASES BY THE ACTION OF EONTGEN HAYS AND OTHER AGENTS. 
448 
negative fogs last for two or three seconds. This is due to the “ electric wind ” accom¬ 
panying the positive discharge being the stronger. Possibly the greater expansion 
recpiired to give visible condensation when the wire is positive may be due to this rapid 
motion of the contents of the cloud-vessel causing very thin fogs to be overlooked. 
Except under certain conditions, to be mentioned immediately, the expansion 
required to catch the nuclei in hydrogen shows very little diminution (none in the 
case of the positive discharge) when the discharge is stopped by short-circuiting the 
terminals before the exnansion is made. A slight effect of this kind can be detected 
when the wire is negative, but the least expansion required to give even a slight 
shower, whatever interval might be allowed to elapse before making the expansion, 
was not less than that corresponding to a gauge-reading of 13 centims. The difference 
between air and hydrogen in this respect is not entirely due to the more rapid 
diffusion of the nuclei in the latter causing them to reach the sides of the vessel 
before they have time to grow to any considerable extent, for nuclei, requiring a 
pressure fall of more than 15 centims., can be detected even 30 seconds after the 
discharge has been stopped, and when this interval only amounts to 15 seconds they 
are sufficiently numerous to give quite a dense fog on expansion. Possibly even this 
slight tendency to become larger exhibited by the nuclei produced in hydrogen when 
the discharge from the pointed wire is negative, is really a remnant of the effect now 
to be described. 
When the apparatus was first charged with hydrogen, fogs could be obtained under 
the conditions just described (after the discharge was stopped), even with 
comparatively slight expansions. The effect was much more marked with the 
negative than with the positive discharge, and was often absent in the latter case. 
If, however, the experiments \yere continued for a day or two the fogs obtained under 
these conditions became gradually less dense, and finally only a few drops could be 
olkained with expansions less than that corresponding to a jDressure fall of 15 centims. 
There can be little doidffi that the effect just described is due to some impurity, 
probably air or oxygen remaining in the apparatus, either mixed with the hydrogen, 
or absorbed by the platinum wire. This is gradually removed by combination witli 
the hydrogen, the combination l.'eing doubtless hastened by the luminous discharge 
from the point of the wire. 
We may conclude from the condensation plienomena attending the discharge of 
electricity from a pointed platinum w^ire, that in a discharge of this kind, whether in 
ions 
identical with those which are 
air or hydrogen, the electricity is carried 
produced in air exposed to Eontgen rays. 
The after-effect of the discharge, noticed in air and under certain conditions in 
hydrogen, is probably a consequence of the chemical combination which can scarcely 
fail to take place at the glowing point of the wire ; where also the ions are, doubtless, 
liberated. So long as the difference of potential is maintained high enough to 
produce the discharge, the carriers are driven across to the walls of the vessel, before 
