692 Dr. G. Johnstone Stoney on 
helium in those springs: but with this marked difference, 
that whereas the other gases are present in such proportions 
as are consistent with their merely being portions of those 
gases which are being returned to the atmosphere after having 
been washed down into the earth from the atmosphere by 
rain, the case is entirely different when we come to helium. 
The quantity of helium passed into the atmosphere through 
those springs, 1s found to be from 3000 to 6000 times more 
than can be accounted for as a return to the atmosphere of 
helium which had been washed down out of it. Accordingly 
we are justified in regarding this great surplus of helium as 
being an addition which is being uninterruptedly made to the 
atmosphere. Notwithstanding this, the quantity of helium in 
the atmosphere has not gone on increasing. The earth at 
the present rate of supply furnishes in a small number of 
years a quantity of helium equal to the quantity which the 
atmosphere can at present retain,—. e. in a number of years 
which is exceedingly small from a geological standpoint, 
which is the point of view that is here appropriate. The 
inference from these facts is the obvious one, that helium is 
by some agency being eliminated from our atmosphere as 
fast as it is being introduced into the atmosphere from the 
earth. Two possible agencies for the elimination of the 
helium suggest themselves—chemical reactions; and an escape 
of helium from the upper part of the atmosphere. Of these, 
chemical agency is excluded by the extreme chemical inert- 
ness of helium, What remains then is that there is an outflow 
of helium from the top of the atmosphere equal to the inflow 
at the bottom, and that the trace of helium which is at any 
one time present in the atmosphere is helium part of which 
is slowly making its way upwards to the situation from which 
some of its molecules can escape, and so produce that outflow 
which balances the net influx at the bottom of the atmosphere. 
Having satisfied myself that the deductive method as I 
applied it (and as Mr. Cook has applied it) lands us in 
erroneous results, I set to work to scrutinize the data of the 
deductive argument with a view to ascertaining how far they 
may be depended upon, and at what points they are doubtful. 
Ail branches of physics require us to be more or less on our 
guard against trusting without sufficient scrutiny to inferences 
from that mixture of theory and hypothesis of which we are 
obliged to make use in order to be able to employ mathematics 
in physical research. The demand for this caution becomes 
a pressing one when, as in gases, we are obliged to deal with 
immense numbers of events, each of which has its own 
dynamical history with incidents peculiar to itself, and where 
