GASES BY THE ACTION OF EONTGEN EAYS AND OTHER AGENTS. 
447 
It still remains possible that the comparatively few nuclei, all requiring large 
expansions, which can be detected in hydrogen exposed to strong ultra-violet light, 
may consist of ions produced throughout the volume of the gas. This point could 
easily be tested by experiments like those just described. 
Further experiments were made with air exposed to the much more intense rays 
which were obtained when a quartz lens was interposed between the zinc points and 
the quartz plate. The intensity of the ultra-violet rays was then sufficiently great to 
give fogs with comparatively slight expansion. The apparatus being arranged to give 
expansion corresponding to a gauge reading ^9 = 64 millims., no condensation (in the 
absence of electromotive force) was obtained with an exposure of 10 seconds, while 
an exposure of 15 seconds with an expansion of the same amount gave a fog. The 
nuclei thus took between 10 and 15 seconds to grow large enough to be caught with 
the degree of expansion used. Yet the application of a difference of potential of 240 
volts between the plates did not prevent very dense fogs being obtained with the 
same expansion with an exposure of 3 minutes. Thus, in spite of the electric field, 
the nuclei were able to exist for more than 10 seconds ; in other words, they took more 
than 10 seconds to travel across the space between the plates, which were again 1‘6 
centims. apart. They thus took more than 1000 times as long as the nuclei produced 
by Kontgen rays to travel the same distance. It is, therefore, veiy inqji'ohaljle that 
the growth of the nuclei under the action of strong ultra-violet rays, or the diminution 
of the expansion required to catch tliem, is the result of any electrification of the 
nuclei by the action of the rays. Another explanation, therefore, than the possession 
of a charge of electricity by the drops, must also be sought for the persistence of the 
visible fogs, which are the final result of prolonged exposure to sti'ong ultra-violet rays. 
The great diminution of the number of drops which are pi'oduced on expansion 
when an electromotive force is applied dniing the exposing of" the air to Kontgen 
rays is easily understood. Foi' tlie number of nuclei present at any instant is 
proportional to the rate at which these are heing produced l)y the rays and to the 
average length of time for which they persist. Now before the application of the 
electric field the average life of the nuclei, depending on the rate at which they 
combine with one another or reach the walls l^y diffusion, is seen to Ije something of 
the order of 1 second, for a large proportion of the nuclei persist for 2 or 3 seconds. 
Now, when the electric field of the intensity used in the experiments is applied the 
time for which they persist must, as has been seen, Ije reduced to something like xo 
part of this. The numljer of drops in tlie fog wTll be diminished in the same ratio. 
The immense difference in the appearance of the fogs with and without the action of 
the electric field is in com23lete agreement with this. The conqffete absence of any 
such dillerence in the case of the fogs produced under the action of the ultra-violet 
light is again a proof that the nuclei on which their production depends do not move 
under the action of an electromotive force. 
