230 
PROCEEDINGS OP THE PERTHSHIRE SOCIETY OP NATURAL SCIENCE. 
Though a volcano may eject much matter in the form of j 
lava, and smoke, and steam, we must remember that it is 
with its most finely-divided products that we have now to 
do—namely, that which is capable of remainicg suspended 
in an atmosphere approaching vacuum. That one erup¬ 
tion, though it may have had 50 vents, should have thrown 
up such quantities in a few days, and that it should at 
once have been diffused all over the outer area of the 
earth’s atmosphere, is simply inconceivable. 
Whilst we see formidable objections to the volcanic 
dust theory, it must be confessed that some of the objec¬ 
tions advanced against it are hardly valid. A gentleman 
resident in the Transvaal states that he was led to abandon 
it, because the glow continued as splendid as ever after 
long-continued and heavy rain which had prevailed in that 
region of Africa. He beiieved that the dust could not be 
the cause, since the rain must have thoroughly washed it 
out of the atmosphere ; but he forgot that the theory sup¬ 
poses the dust to float far above the region of the rain- 
clouds. These never rise above 7 or 8 miles, whereas the 
theory requires the dust to float at four times that eleva¬ 
tion. At such altitudes there could be no rain to wash it 
down. Eor these reasons, we fear that the volcanic dust 
theory must be abandoned. But another has been ad¬ 
vanced, which appears to us to merit no lengthened consi¬ 
deration. The glow has been ascribed to the existence of 
aqueous vapour in the higher regions of the atmosphere; 
but how it can reach the regions in which it must exist to 
produce the effects witnessed, and how it could continue, 
even if it reached them, is not explained. Aqueous vapour 
would undoubtedly produce the refractions and reflections 
the sun’s light would require to undergo to produce the 
glow, because it is by it that the frequent glories of an 
ordinary sunrise and sunset are occasioned. But to pro¬ 
duce the true phenomena of the glow,—to occasion that 
which made it different from an ordinary sunset or sun¬ 
rise,—the aqueous vapour would require to be extended 
prodigiously upward, even to regions where we have no 
reason to suppose it could by any possibility exist. 
Aqueous vapour is sublimated into the air from the surface 
of the sea and from the moist earth by the heat of the 
sun. It can remain suspended only in comparatively 
warm air,—by no possibility can it continue as vapour in 
intense cold. One diminution of temperature reduces it to 
rain, and a lower to ice and snow. Now, we know from 
the testimony of our mountain-climbers and aerial voyag- 
eurs that, as we ascend into the atmosphere, the cold 
rapidly increases, until, at the elevation of a few miles, it 
becomes unendurable by human beings. The breath falls 
as snow from the mouth and nostrils at 5 miles up, just as 
it does in Melville Island during the depths of an A-rctic 
winter. Even under the Equator the snow-line is only a 
few miles above the sea-level. Vapour of an aqueous 
kind in the higher regions of the atmosphere is, therefore, 
an absolute impossibility,—it can no more exist than snow 
could in a heated furnace. In ascending upward from the 
earth’s surface, we soon rise completely beyond the region 
of the clouds, and reach spaces where the atmosphere is so 
cold and attenuated and still, that every kind of life, and 
matter in any palpable form, can hardly be conceived of 
as existing :—and as we ascend, the cold and attenua¬ 
tion and silence increase, until we reach a region 
where the atmosphere, from its infinite attenuation, 
ceases altogether to exist, and absolute vacuum be¬ 
gins. That point has not with minute scientific accuracy 
been determined, but it is generally supposed to range from 
40 to 60 miles,—although so high an authority as Dr Ball 
believes that it may be more than a hundred. Indeed, the 
boundary line—if we may so call it—must be continually 
changing, since there is a daily tidal wave in the ocean of 
the atmosphere, similar to, and produced by the same 
causes as, that in the sea;—and beyond this slight daily 
change, there are gigantic atmospheric waves, occasioned 
by the unequal heating of the earth’s surface by the sun, 
and other causes, which sweep round the globe, the 
varying depths of which we are enabled to read by the 
fluctuations of the barometer. Still, making allowance for 
these changes, the boundary between absolute vacuum and 
the atmospheric surface is usually confined between these 
extremes we have mentioned. Now, since, as we have 
shown, the matter occasioning the glow must exist in these 
extremely exalted regions of the atmosphere; and since, as 
we have farther shown, neither solid matter, however 
minute, or aqueous vapour in any form, could remain 
suspended in them, the question arises—By what was the 
glow occasioned 2 How is its existence for a definite 
period to be accounted for ? 
The theory we have to propose is, that the sun and his 
attendant planets, in their grand flight southward in the 
direction of the constellation Hercules, passed through one 
of those nebulous or cometic fields, or masses, which we 
have reason to believe exist in the interstellar spaces. 
That the atmosphere of the earth coming in contact with 
such matter, and in its upper regions being permeated or 
blended with it, would be in the condition to produce the 
phenomena of the glow, will, we think, be readily granted. 
So that the main point in our endeavour will be to show 
that such nebulous or cometic masses most probably exist 
in the vast fields of space around us, and that it is not im¬ 
probable that our system during the continuance of the 
