248 
NA LORE 
[JANUARY 14. 1904 
would sufficiently represent what goes on in the actual 
gases with which we have to deal. This expectation, un- 
fortunately, has not yet been fulfilled (see Proceedings of 
fhe American Philosophical Society, vol. xlii. p. 108). 
(2) The problem has been treated inductively by arguing 
upwards from known facts of nature. It is in reference to 
this second method that the Times makes its statement. 
So far from its being true, as was supposed by the Times, 
that the argument is based on the assumption that there is 
no helium in our atmosphere, it is pointed out in the first 
memoir upon the subject that there must be just such traces 
of helium and hydrogen in our atmosphere as have since 
been detected (see Scientific Transactions of the Royal 
Dublin Society, vol. vi. pp. 308 and 309). 
The facts of nature which were made the data of the 
investigation are four in number. The first of these is that 
there is either no atmosphere or very little on the moon, 
from which it is inferred that the atmosphere which the 
moon shared with the earth when the two bodies separated, 
-and whatever atmospheric gases have since been evolved 
upon the moon, have by this time escaped. It can be shown 
that if this be so, then hydrogen, if uncombined, must be 
able to escape from the earth. There is, however, but little 
free hydrogen upon the earth, and in the atmosphere there 
is only the merest trace. If there is in this trace any excess 
over what returns to the earth in rain or in other ways, this 
excess is on its way upwards towards the penultimate 
stratum of the atmosphere, which is the part of the atmo- 
sphere from which gases escape. Accordingly, the amount 
of hydrogen which succeeds in getting away from the earth 
must be very small, while the store of hydrogen locked up 
in the ocean and in the solid earth is enormous. It can, 
moreover, be shown that there is a minute accession of 
hydrogen to the earth from outside, so that on the whole 
the quantity of hydrogen upon the earth may be almost 
stationary. 
The second and third facts used as data are that helium 
and free hydrogen are being continuously supplied from the 
earth to its atmosphere, and that—probably in both cases, 
certainly in the case of helium—only a very small _per- 
‘centage of the gross supply is being washed down by rain 
or in other ways returned to the earth, notwithstanding 
which neither the hydrogen nor the helium has gone on 
accumulating in the atmosphere. From this it is inferred 
that the quantity which is present in the atmosphere has 
adjusted itself to be such that the outflow of these gases 
from the upper regions of the atmosphere balances the net 
supply which the atmosphere receives from below. 
One other fact in nature is used as a datum—that the 
earth’s potential of gravitation is sufficient to prevent any 
sensible escape of the lightest of the abundant constituents 
of its atmosphere. This lightest abundant constituent is 
the vapour of water. 
A further paper has been published which is devoted 
specially to dealing with the behaviour of helium in the 
earth’s atmosphere (see Astrophysical Journal, vol. xi. p- 
369). In this paper it is shown from the marvellously 
accurate determinations made by Sir william Ramsay and 
his assistants that the supply of helium to the atmosphere 
by hot springs, and presumably the helium which oozes up 
elsewhere through the soil, is from 3000 to 6000 times more 
than can be accounted for as being a return to the atmo- 
sphere of helium which had been washed down by rain; 
whereas the argon, oxygen and nitrogen in such springs 
are all of them present in proportions which are consistent 
with their having been carried down by rain from the atmo- 
sphere. From which it is inferred (1) that nearly the whole 
of the small quantity of helium in the atmosphere is on 
its way outwards; (2) that helium would have become a 
larger constituent of the atmosphere by reason of the influx 
from below if there had been no simuitaneous outflow from 
above ; (3) that the rate of this outflow is presumably equal 
to the net rate of supply. ] 
The escape of helium from a member of the solar system 
must be facilitated by the circumstances that those radi- 
ations from the sun that can affect helium have the full 
strength of radiation from the photosphere, inasmuch as 
the helium in the sun’s outer atmosphere emits radiations 
of the same intensity as the photosphere. This is evidenced 
by the great helium line D, being as bright as the neigh- 
NO. 1785, VOL. 69] 
bouring part of the spectrum of the photosphere. We have, 
moreover, to take into account that outpour of corpuscles 
from the sun which, in the upper regions of our atmo- 
sphere, is able to excite into intense activity the internal 
motions of krypton which produce the green auroral line, 
and presumably with equal and perhaps increased vigour 
imparts energy to the molecules of helium which range to 
still greater altitudes. 
G. JOHNSTONE STONEY. 
30 Ledbury Road, W., January 7. 
On the Origin of Spiral Nebulas, 
Tue ever increasing interest and importance of studies 
relating to celestial phenomena naturally lead up to ques- 
tions which, in the present state of our knowledge, can 
(from the purely theoretical standpoint) in some cases be 
answered in a fairly satisfactory way. 
The object of this note is to present certain views (some 
of which are believed to be new) on the probable origin of 
spiral nebulas, having given to start with an incandescent 
body like our sun. 
From theory and observation we know that when different 
parts of the same fluid body have largely different tempera- 
tures the mass is in unstable equilibrium. The constant 
tendency of the resulting flow of the fluid is to equalise the 
temperature throughout the mass. 
If the maximum temperature is in the interior of the 
body and the outside is exposed to a much lower tempera- 
ture, the flow near the surface, through a gradual congeal- 
ing of the latter, will be retarded. Such a surface will 
then also act as an insulator and shield to prevent both the 
too rapid loss of internal heat and the free escape of the 
accompanying gases. 
The visible photosphere of the sun is known to be in a 
highly heated condition, and the fact that it is almost con- 
stantly being ruptured (in some zones more strongly and 
frequently than in others) shows—reasoning from analogy 
—that the solar surface has the properties of a fluid in 
such a state of unstable equilibrium that the superheated 
confined masses in the interior are still able to break through 
this surface at many points. Z 
If the sun did not rotate on an axis, this surface would 
probably be of uniform strength throughout, for the interior 
circulation would ther be radial. The resultant, however, 
of the rotary and radial forces acting on each particle pro- 
duces not only an ellipsoidal figure, but also has the 
tendency to cause each ascending particle to move towards 
the equator. 
As a result there is a tendency to produce’ surface-flow 
towards the equator causing an accumulation of cooled 
matter along the zone which but for this flow would be 
the weakest part of the whole rotating surface. It is there- 
fore to be expected that two zones of least strength should 
exist in the solar surface, symmetrically situated with refer- 
ence to the equator, but at some distance from it. 
Now what is most likely to happen after a body like the 
sun has contracted to such a radius that the surface exists 
in the plastic or semi-solid state? 
Such a surface will act as an insulator producing a more 
nearly uniform internal temperature and a consequent de- 
crease in the interior circulation. The surface flow having 
ceased, and the axial velocity of rotation having increased, 
the zone of least surface-strength will coincide with the 
equator. 
During the time required to reach this stage of the body’s 
history it is probable that the lesser vents were gradually 
closed as the surface became stronger, resulting in periodic 
outbursts of increasing magnitude at a smaller number of 
openings until finally these also were closed. 
As the weight of each particle of matter in the surface 
has increased inversely as the square of the radius (the 
sun’s radius being unity), the internal pressure has been 
increased. Through. the continued. contraction of the 
outer surface this pressure, no longer relieved by periodic 
outbursts, increases far beyond the limit necessary to sup- 
port the surface; as a result, the outer boundary grows 
hotter and consequently weaker, so that at last a great © 
rupture of the surface takes place on or near the equator. 
The moment this break occurs, the interior masses and 
