GASES BY THE ACTION OF BONTGEN BAYS AND OTHEB AGENTS. 
423 
No effect could be obtained with solid KOH or strong H 2 SO 4 in the tube. 
With aqueous H2SO4 containing 45 per cent, of H0SO4, corresponding to a 
relative humidity of about 50 per cent., no fog was obtained. Over 10 per cent, 
sulphuric acid, which had been all night in the tube, so that there can be no doubt 
that the equilibrium vapour pressure was reached, a fog very quickly appeared under 
the action of the ultra-violet light. Over aqueous caustic potash, containing about 
17 per cent, of KOH, a fog was readily obtained. The relative humidity over such a 
Fig. n. 
solution is less than 90 per cent. Experiments have not l^een tried with humidity 
between 50 and 90 per cent. These experiments then show that Ijoth air and water 
vapour are necessary for the production of the ultra-violet light fogs; it is not 
necessary that the air should he saturated. 
The cloud particles produced by the action of ultra-violet light persist, for some 
hours at least, after the rays have been cut off. This is so, even when the air is 
unsaturated; for example, the fog produced over a 17 per cent, potash solution was 
found to be still visible three hours after the arc was stopped. The drops are 
therefore small enough to settle with extreme slomiess ; yet in spite of their small 
size there is no indication of any tendency for them to evaporate again. It is 
probable, therefore, that the drops do not consist of pure water. We might, it is 
true, account for their persistence by supposing each to have become charged with 
electricity under the influence of the ultra-violet rays. In the light of latei- experi¬ 
ments, however, the former view appears to he the more probable. 
Any discussion of the nature of these fogs is, however, postponed till the experi¬ 
ments made with other gases than air have been described. 
Before going on to describe the exj^eriments made wdth oxygen, mention should be 
made of experiments in which no indiarubber or cement of any kind came in contact 
witli the air exposed to the ultra-violet rays. For this purpose a test-tube Avitli the 
open end ground smooth was closed by a plano-convex quartz lens, simply held in 
position by an indiarubber band, no indiarubber washer Ijeing inserted. The inside of the 
tube was moistened with distilled water; the air inside was at atmospheric pressure. 
The tube was fixed in a horizontal position and left for two days to allow the dust 
particles to settle. On exposing to the light of an arc lamp, placed so that its light 
