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Proceedings of the Royal Society of Edinburgh. [Sess. 
perhaps properly to come in under the effects of other gases on S0 2 . When 
testing the effects of emanations the flask E is replaced and the bottle 
(without its stopper) containing the radio-active substance is placed in it, 
so that the emanations may escape and be carried with the filtered air into 
the flask S and there mixed with S0 2 . Most of the substances tested gave 
very slight condensation, and the emanations from the radium salt, 
which gave dense condensation by the action of its /3 and y rays, gave 
only a very slight effect. The only substance tested that gave a strong 
action was a thorium hydroxide. A bottle containing a small quantity of 
it, placed in the flask E, gave a constant and a dense condensation in the 
test-flask. In this case it is the a rays that are responsible for the very 
dense fogging. The same bottle of thorium seemed to have little effect in 
discharging an electroscope, but its emanations discharged it quickly. The 
probable reason for the great difference between the radium and the thorium 
emanations is that there was much more of the latter salt than of the 
former, and also that thorium emanations break down about 6000 times 
more quickly than the radium. In these experiments with thorium emana- 
tions it is not necessary to use the flask E. The bottle with the thorium 
may be put in a flask connected with the outer end of the filter, when 
the emanations are drawn into S through the filter. The emanations 
come quickly through the cotton-wool — that is, they are but little 
absorbed by the filter, and they can be quickly washed out with 
air; unlike most gases, the first part of which is all absorbed and it is 
hardly possible to wash them all out afterwards by continued pumping, 
while the emanations, being inactive, pass freely through. The action of 
the radio-active bodies is probably due to their ionising properties, but the 
nuclei which cause spontaneous condensation are probably formed by the 
oxidation of the S0 2 by the peroxide of hydrogen formed by the /3 rays in 
the moist air, and also by the action of a and j3 rays converting the 
oxygen into ozone. 
Neither the radio-activity of the atmosphere nor the emanations from 
the ground seem likely to play any part in these sun-fogs, as they are 
always present night and day. With regard to the possible radio-activity 
of the sun, we require more information before we can say whether or not 
it plays any part in the phenomenon. It is not likely that either the a or 
the {3 rays can penetrate through our atmosphere; and as we can keep 
photographic plates with only a covering of black paper, the y rays cannot 
be of much importance, even if they exist. 
