690 Dr. G. Johnstone Stoney on 
plates, 7. e. the maximum distance fallen, was 4:7 cms.; the 
rate of fall when negative ions alone came into action thus 
did not exceed 1°6 cm. per second. Treating these data in 
the manner described by Professor Thomson*, we find that 
the number of negative ions present, a the absence of an 
electric field, is less than 1000 per c.c.: this is in agreement. 
with the vaiue calculated above from the data afforded ox 
leakage experiments. 

LXXVI. Escape of Gases from Atmospheres. 
To the Editors of the Philosophical Magazine. 
GENTLEMEN, 
LETTER under the above heading, by Mr. 8. R. Cook 
in ‘ Nature’ of the 24th of March, ‘puts forward views 
which ought not to remain on record without reply ; and as 
between 30 and 40 years ago I carried on the investigation 
into the rate at which gases can escape from atmospheres in 
the same way as Mr. Cook has done, and arrived from the 

premisses employed by him at. substantially the same con-. 
clusions, perhaps the best answer will be to state the 
considerations which led me to distrust that line of argument, 
and finally to abandon it. To do this, however, requires more 
to be said than can be brought within the compass of a letter 
to a weekly journal; and on this account, and because the 
discussion is a phy rsical discussion and concerns one of 
nature’s greater operations, [ venture to request for the 
following } pages the hospitality of the Philosophical Magazine. 
A study of the phenomena attending the escape of gases 
from atmospheres has been approached in two ways— 
imductivelyt, by arguing upwards from events which are 
found to have occurred or to be in process of occurring in 
nature; and deductively t, by drawing inferences from the 
supposition that it is legitimate to attribute to the real gases 
of nature, behaviour which it has been ascertained would 
prevail in certain models of gas, so much simpler in their 
* ‘Electrical Properties of Gases,’ p. 121. 
+ “Of Atmospheres upon Planets and Satellites.” By G. Johnstone 
Stoney, F.R.S. See Scientific Transactions of the Royal Dublin Society, 
vol. vi. p. 805 (October 1897); or Astrophysical Journal, vol. vii. p. 25 
(January 1898). . 
t “On the Escape of Gases from Planetary Atmospheres according to 
the Kinetic Theory.” ByS. R. Cook. See Astrophysical Journal, vol. xi. 
No. 1 (January 1900). 
“'lhe Kinetic Theory of Planetary Atmospheres.” By Professor G. 
H. Bryan, F.R.S. See Philosophical Transactions, A. vol. exeyi. p. 1 
(March 1900). 
