PRESIDENTIAL ADDRESS. 129 
and interstellar space, we early looked in their spectra for 
the prominent nebular, coronal, and auroral rays.”’ 
It is a source of satisfaction to have the presence of free 
hydrogen in the air demonstrated at the same time by two 
such different processes; and, now that its presence in the 
atmosphere is assured, another old and interesting question 
has been reopened, as to the relative proportions of the mixed 
‘constituent gases of the atmosphere as higher and higher 
elevations are reached, and as to whether hydrogen can 
escape from the influence of the earth’s attraction. Dr. 
' Johnstone Stoney considers it impossible that the earth can 
retain by gravitational attraction either the hydrogen or the 
helium of the atmosphere, in which case the earth would be 
undergoing a steady loss of these gases. On the other hand, 
Liveing and Dewar argue that “if the earth cannot retain 
hydrogen, or originate it, then there must be a continued 
accession of hydrogen to the atmosphere (from interplane- 
tary space), and we can hardly resist the conclusion that a 
similar transfer of other gases must also take place.” 
The settlement of this question by an appeal to experi- 
ment must necessarily be difficult; some might be ready to 
say impossible. The theoretical conclusions depend upon 
considerations purely physical, and the chemist might yet 
be able to point out a natural process at work sufficient to 
meet the whole difficulty, and by which these gases are com- 
bined—the combustible gases at least—oxidised, and retained 
for the earth. We have abundance of oxygen; we have 
a powerful oxidising atmospheric ingredient; and we have 
atmospheric electricity, together with solution during pre 
cipitation. . 
According to Hinrichs, if we assume each constituent gas 
to form an independent atmosphere (Dalton), and that there 
is no reciprocal action, then, by calculation (La Place’s for- 
mula), at 6-2 miles high the hydrogen will have increased 
from 9-02 to 0-06 per cent.; at 18-6 miles, the carbon 
dioxide will have disappeared, the hydrogen increased to 
0-64 per cent., and the oxygen lowered to 13-9 per cent. ; 
at 37-2 miles, argon will have reached its limit, the hydrogen 
constitutes 16-94 per cent., and the oxygen have fallen to 
7°52 per cent.; at 62 miles high, the hydrogen equals 95 
per cent., the oxygen falls to 0-3, and the rest is nitrogen— 
all being per cent. by volume. 
If the percentage of hydrogen in air is, under ordinary 
conditions, anywhere between 5 per cent. and 72 per cent., 
the mixture explodes when fired with a light. We might 
expect explosive mixtures in the higher regions of the atmo- 
K 
