Sroner—Of Atmospheres upon Planets and Satellites. 309 
The evidence that there is an escape of gas from the Earth’s atmosphere is 
still more conspicuous in the case of helium. Small quantities of this gas are con- 
stantly being dribbled into the atmosphere by hot springs and probably in other 
ways, and it was probably supplied more copiously in former times. Now helium 
is so little disposed to enter into combination with other elements, that the 
efforts of chemists to effect any such union have been unavailing. We must 
conclude, therefore, that this gas remains unchanged within the atmosphere, 
where it would therefore, in the lapse of time, have accumulated so as to be now 
a sensible and perhaps a large constituent of the Earth’s atmosphere were it not 
that it is escaping from the atmosphere’s outer boundary as rapidly as it enters it 
below—indeed so promptly escaping, that the amount i fransitu is too small for 
the appliances of the chemist to detect it. 
On the other hand, water is not sensibly leaving the Earth. From which we 
learn that the potential of the Earth and the temperature at the boundary of its 
atmosphere are such as enable our planet effectually to imprison the vapour of 
water with molecules whose mass compared with molecules of hydrogen is 9 
(and probably ammonia with a density of 8°5). The other constituents of the 
Earth’s atmosphere, such as nitrogen, oxygen, and carbon dioxide, have still 
heavier molecules. Accordingly, none of these escape in sufficient numbers to 
produce any perceptible diminution of the quantity of gas upon the Earth.* We 
may infer from this that the boundary between those gases that can effectually escape 
from the Earth and those which cannot, lies somewhere between gas consisting of molecules 
with twice the mass of molecules of hydrogen and gas with molecules whose mass is nine 
timest the mass of molecules of hydrogen. 
This we may take to be one fact which we can ascertain by observing what 
occurs upon the Earth, and the telescope has been able to reveal to us another fact 
of alike kind, viz. that there is either no atmosphere upon the Moon, or excessively 
little—a fact which has been made certain by the application of very delicate tests. 
* We need not suppose that there is absolutely no escape of the molecules of the denser gases, but 
only that the event is an excessively rare one. Thus, if the molecules of a gas escape so very seldom 
that only a million succeed in leaving the entire atmosphere of the Earth in each second, then a simple 
computation will show that it would take rather more than thirty millions of years for a uno-twentyone 
(the number represented by 1 with 21 cyphers after it) of these molecules to have escaped. Now a 
uno-twentyone is about the number of molecules which are present within every cubic centimetre 
of the gas at such temperatures and pressures as prevail at the bottom of our atmosphere. An escape 
of molecules of the denser constituents of the atmosphere on this excessively small scale, or even on a 
scale considerably larger, may be and probably is going on. See a paper on the ‘ Internal Motions 
of Gases” in the Philosophical Magazine for August, 1868, where the number of molecules in a gas 
is estimated. Readers of that paper are requested to correct a mistake at the end of the third paragraph, 
where 16? was by an oversight inserted instead of af 16, 
+ We shall find in the Chapter on Venus that the presence of water on that planet enables us 
to somewhat lower the upper of these two limits. 
