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Escape of Gases from Atmospheres. 691 
constitution than real gases that the progress of events within 
them is susceptible of mathematical treatment. 
The two methods, as hitherto employed, have led to con- 
tradictory results, of which one at least must be erroneous. 
Mr. Cook, who has of recent years employed the deductive 
method, expresses the opinion in his letter that the numerical 
results which have been arrived at by this method “ will have 
to stand” until they can be disproved “by other a priori 
reasoning.” Serious students of nature must, I think, hold 
that man, in his dealings with nature, is not in a position 
to limit in this way the kind of proof he will accept; and 
that it is sufficient if in any way Mr. Cook’s inferences from 
Maxwell's researches can be disproved, whether by valid 
a priori or by valid a posteriori reasoning. And, moreover, 
that when once they are disproved we are brought face to 
face with the fact that there has been a mistake somewhere 
in the data which have led those who trusted in them toa 
false conclusion. 
What convinced me several decades ago that the conclusion 
at which I arrived, and at which Mr. Cook has arrived, is 
false, is that it represents the moon as incompetent to get rid 
ot the atmosphere which it originally shared with the earth, 
and of the gases which it has since evolved in abundance 
from its own interior. We knew 35 years ago, as we know 
now, that any reasoning which makes out that the moon has 
retained its atmosphere, must have a flaw in it somewhere. 
Furthermore, since that time, other facts not then known have 
come to light, and in a mar ked degree confirm the judgment 
which was then formed. Our confidence that we are on the 
right track is justifiably strengthened when, as in this case, 
further discoveries as they emerge confirm the view to which 
we had been led when our materials were more scanty. The 
presence of helium on the earth was not then known: and 
the argument* which has been based on what is now known of 
its behaviour may be summarised as follows :—Helium is 
supplied to the earth’s atmosphere through certain hot springs, 
and under circumstances which indicate that it also oozes up 
through the soil. It is, however, what is carried up by the 
water of these springs that can be subjected to experimental 
examination. The other gases of our atmosphere, such as 
nitrogen, oxygen, and argon, are found to accompany the 
* The argument here summarised is based on the marvellous determi- 
nations made by Sir William Ramsay, K.C.B., F.R.S., or in his laboratory, 
and will be found with the necessary “details i ina paper “ Onthe Behaviour 
of Helium in the Earth’s Atmosphere, ” by G. Johnstone Stoney. See 
Astrophysical Journal, vol. xi. p. 369 ( 1900). 
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