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atmosphere ; for, as has been already shown, the change in temperature on
the comets is incomparably greater than it is on the earth, and its variation
corresponds with the variation in the atmosphere of the comet.

Angstrom has also shown that the light from the aurora, the corona, and
the zodiacal light are all of the same character, or all give the same bright



12 ON THE TAILS OF COMETS, THE SOLAR CORONA, AND THE AURORA. [2

lines when viewed through the spectroscope, and that these lines correspond
to the light from no known substance. This indicates that, whatever this
light may be, the incandescent material is the same in all cases ; or may we
not assume that it is the medium which fills space, that is illuminated by the
electrical discharges ? This would be supported by the fact that the light
from the heads of two small comets indicated carbon, whereas that from the
tails only gave a faint continuous spectrum. For an electrical discharge
would first illuminate the atmosphere of the comet, or even carry some of the
solid material off in a state of vapour, and then pass off to the surrounding
medium ; thus, while the spectrum from the head would be that of cometary
matter, the tail would be due to the incandescent ether.

I would here suggest that gas, when rendered incandescent by electricity,
may reflect light ; (it will certainly cast a shadow from the electric light ;)
and if this be the case, part of the light from comets' tails may after all be
reflected sunlight.

At any rate, it is certain that the appearance of streamers, the rapidity of
change and emission, the perfect transparency, and the wave-like fluctuations,
which belong to these phenomena, are all exhibited by the electric brush ; in
fact the electric brush will explain all these appearances, which have defied
all attempts at explanation on a material hypothesis.

I have only to add that the main assumption involved in the electrical
theory is, that space is occupied by matter having similar electrical properties
to those of gas ; and I would ask, is it not more rational to make such an
assumption, than it is to attribute unknown and inconceivable properties to
cometary matter ?

Theories, even, if founded only on rational speculation, often, I believe,
prove very useful, inasmuch as they afford observers a definite purpose in
their speculations something to look for, something to establish or to refute ;
and I publish these speculations of mine at this particular moment in the
hope that they may perchance serve such a purpose.



PART II.

(Read February 7, 1871.)

In the paper which I read before this Society on the 29th of November
last, I endeavoured to show that it is probable that these phenomena are a
species of that action, known as the electric brush, taking place in the medium
which fills space, be it ether, or simply gas, or both. The reasoning I made
use of was essentially a fortiori. I pointed to the fact that the electric brush
as seen in the Geissler tubes exhibits similar appearances, and that at the



2] ON THE TAILS OF COMETS, THE SOLAR CORONA, AND THE AURORA. 13

times of greatest display on the part of comets and the aurora similar conditions
are present, such as a change in the action of the sun, conditions which, to
say nothing more, are favourable to electric disturbance. I purposely avoided
all attempts to explain how the brush may be produced, feeling that it was
sufficient to point to the aurora, which is universally admitted to be electrical,
as a proof that such phenomena do exist, even if we cannot explain how.
This proof, however, is perhaps not quite satisfactory. In order that it might
be complete, the other phenomena would have to be produced in the same
way as the aurora ; and this, although possible, is not necessary. An
assumption, which is commonly made respecting the phenomena of the aurora,
cannot be made with respect to the others. This assumption assigns the two
magnetic poles of the earth as the two electrodes, between which the electrical
discharge takes place, which forms the aurora borealis and the australis. If
this assumption be maintained, some other explanation must be found for the
manner in which electricity may form the tails of comets and the corona. It
is quite clear that the tail of a comet cannot be due to a discharge between
two electrodes situated on the comet itself. In the same way, from the
position occupied by the corona, it can hardly be due to electricity passing
between two electrodes on the sun. In fact, if a comet's tail is electrical, it
is due to a discharge of electricity, of one kind or another, from the comet,
which for the time answers to one of the electrodes only. The same may be
said of the corona and the sun. If we could observe the aurora from a point
distant from the earth, it is very probable that we should find the same to be
the case ; but whether this would be so or not, an assumption has been made
as to the cause and nature of the aurora, which will answer just as well for the
corona and comet's tails : it is, that the sun, acting by evaporation or otherwise,
causes continual electrical disturbance between the earth and its atmosphere,
the solid earth being negatively, and the atmosphere positively charged, and
that the aurora is the reunion of these electricities taking place in the
atmosphere.

Now, as has been already said, this assumption will serve for the comets
and the sun, as well as for the aurora. If there is a continual electrical
disturbance between the sun and the medium in which it is placed, so that
the sun becomes negatively, and the medium positively charged, the reunion
of these electricities would form the corona. It must not be supposed that I
assume the sun to be a reservoir of electricity, which it is continually pouring
into space. I consider that the supply of electricity in the sun is kept up by
some physical action going on between the sun and the medium of space,
whereby the sun becomes negatively, and the medium positively charged.

This may be well illustrated by reference to the common electrical
machine : here the motion of the glass against the rubber causes the glass to
become positively, and the rubber negatively charged ; and these electricities



14 ON THE TAILS OF COMETS, THE SOLAR CORONA, AND THE AURORA. [2

do not unite instantly there and then, but remain and accumulate in the
respective bodies, until collected and brought together again by the conductor.

Assume, then, that the sun is in the position of the rubber, while the
ether is in that of the glass ; then the corona corresponds to the spark
or brush which leaves the conductor. On the same assumption, the negative
electricity of the comet would be more and more set free by the inductive
action of the sun, as the comet approached it, and would also be driven off by
induction in a direction opposite to that of the sun and, combining with the
positive electricity in the ether, would form the tail of the comet, in a
manner analogous to that in which a negative spark is given off by the lid of
the electrophorus.

I think that a rational account may in this way be given of the manner
of the electrical action to which I have attributed these phenomena ; but I
do not consider that the probability of the truth of this electrical hypothesis
depends on the value of such an explanation. It is an assumption, based on
the manner in which it fits into its place, and explains the appearances
presented by these beautiful phenomena.

Since this paper was written, my attention has been called to the fact that
Mr Richard Proctor has published views of these phenomena which somewhat
resemble mine. He attributes them in part to electricity, and in part to
meteors. There is, however, this fundamental difference between our views
that he regards the tails of comets as consisting of cometary matter, the
difficulty of conceiving which was the origin of these speculations. Moreover
I can conceive no electrical discharge between two meteors without a
medium between them ; and if there is a medium, why is there any necessity
for meteors ? If, as I see good reason to suppose, gas, when glowing with
electricity, reflects or scatters rather than absorbs light of the wave-length
which it radiates, that portion of the coronal light which is polarized, and
assumed to be reflected, will be accounted for. I think that recent observa-
tions have confirmed the probability of these speculations, inasmuch as they
have confirmed the facts on which these speculations were based. There is
one point which has not been already noticed, but which seems to me to be
of some importance.

If the corona be an electrical discharge, the electricity will be continually
carrying off some of the elements of the sun into space, where they will be
deposited and condensed. May not this stream of matter be the cause of the
existence of small meteors, and supply the place of those which continually
fall into the larger bodies ?



3 A.



ON COMETARY PHENOMENA.



[From the Fifth Volume of the Third Series of " Memoirs of the Literary
and Philosophical Society of Manchester." Session 1871-72.]

(Read November 28, 1871.)

THE observation of comets by powerful telescopes has shown them to be
in a state of violent internal agitation a feature which is as much their
characteristic as tails or vaporous appearance ; for it is not possessed by any
of the planets or fixed stars. Robert Hook seems to have been the first to
notice this: when he observed the great comet of 1G80 (Newton's comet)
through his 14-feet telescope, he saw bright streams issuing from some point
near the centre of the comet's head, and at first taking a direction opposite to
the tail and towards the sun, then gradually diverging, and finally falling
back into the tail. These streams were continually changing in magnitude
and direction, some of them disappearing, and fresh ones appearing in their
places. Their behaviour was such as to lead the philosopher to the con-
clusion that they were flame and smoke, or vapour excited by the action of
the sun on the constituents of the body of the comet. Robert Hook again
noticed this phenomenon in the comet of 1682. In 1836 Bessel observed
similar appearances in Halley's comet ; and, although he appears to have
been in ignorance of the fact that they had been noticed before, he was led
to the same conclusion as Hook as to their origin, viz. that these streams
were jets of vapour caused by the action of the sun's heat on the more solid
part of the comet. This hypothesis, started by Hook and afterwards by
Bessel, seems to have been very generally confirmed by subsequent observa-
tion, almost all comets, large and small, showing signs of the same action.
Now although at first sight there seems nothing improbable in the supposi-
tion that the sun causes a great amount of evaporation on a comet, yet,
before we can admit it as altogether satisfactory, it is necessary to show why
the same action should not go on to the same extent on the earth and on



16 ON COMETARY PHENOMENA. [3 A

the planets ; for neither do the planets show this same appearance, nor are
we aware of any action on the earth which could give rise to these ap-
pearances. I do not know that any attempts have as yet been made to
explain this ; but I think an explanation may be found in the difference
between planets and comets, in their size and the shape of their orbits, in
the fact that the planets are, so to speak, large bodies moving in approxi-
mately circular paths, and so remaining at about the same distance from the
sun, while comets are small and move in eccentric paths, continually altering
their distance from the sun. I have (in the subsequent part of this paper)
endeavoured to state these reasons, and, further, to show that the difference
of evaporation on a comet and on a planet, is a sufficient cause for electrical
phenomena on the former, which do not take place on the latter.

I think that the reason why the materials of comets should at times (i.e.
when the comets are in certain positions) evaporate under the sun's heat in
a greater degree than those of planets, will be rendered clear by considering
the reasons why the heat of the sun does not continually evaporate the
materials of the earth always remembering :

1st, That comets move in eccentric, and planets in nearly circular orbits.
2nd, That comets are very much smaller in mass or weight than planets.

Why, then, does not the heat of the sun evaporate the materials of the
earth ? The heat which the sun is continually pouring into the earth or any
of its surrounding bodies is expended in one of the three following ways :

I. By external radiation from the body.

II. By the evaporation and liquefaction of the materials of the body.

III. By producing changes in the body such as the formation of coal and
the growth of living things.

The amount of heat expended in the third way may be considered very
small in any such body as the earth ; for the amount of energy given out by
the combustion of fuel and the work of animals must be nearly equal to that
stored in the growing plants.

Therefore the heat which the earth receives from the sun during any
period (a day, or a year) is nearly all spent in evaporation and liquefaction, or
radiated away into space. Hence the quantity of heat spent in evaporation
&c., is the difference between the heat received and that radiated away ; and
consequently it follows :

1. If these are equal, there will be no evaporation.

2. If the heat received is greater than that radiated away, there will be
evaporation &c.

3. If less, there will be condensation &c.



3 A] ON COMETARY PHENOMENA. 17

That is to say, if over any definite period of time, the heat which the
earth receives from the sun is equal to that which it radiates into space, then
the amount of ice and vapour will be unchanged (unless there be some inter-
change between these).

If, on the other hand, the heat received is in excess of that radiated away,
the vapour in the atmosphere will increase and the ice diminish, and vice
versa. Now the relation which the heat radiated away bears to that received
will depend on two things, viz. the temperature of the earth's surface, and its
distance from the sun. For both the heat received, and that radiated, depend
in the same way on (and, in fact, are both proportional to) the extent and
nature of the earth's surface ; and the quantity of heat received depends on
(in addition to this) the distance of the earth from the sun (it varies inversely
as the square of this distance); whereas the quantity of heat radiated away
depends on the temperature of the earth's surface, as well as on its extent
and nature. Hence the ratio which the heat received bears to that radiated
away will decrease as the distance of the earth from the sun increases, and
also as the temperature of the earth's surface increases.

If there were nothing to melt or evaporate on the earth (or any other
body whose distance from the sun is nearly constant), then the heat radiated
away would eventually equal the heat received ; for the temperature at the
surface would continually rise until the quantity radiated away equalled
that received, and there was equilibrium. This temperature I have in the
remainder of this paper called the temperature of equilibrium. It will
depend simply on the distance of a body from the sun, increasing inversely
as the square of the distance. Hence in the case of planets this will be
constant, whereas in the case of comets it will vary. If there is any material
on the body, which evaporates at a lower temperature than that of equi-
librium, then there will be evaporation until the material is all gone, or its
conditions of boiling are altered. The temperature at which the softest
material will evaporate, will depend on the nature of that material, and on
the pressure of the atmosphere surrounding the body. Any increase in the
pressure of the atmosphere, will increase the temperature required to
evaporate the material. If initially a body has no atmosphere, then we may
assume that its materials will evaporate, until the vapour forms one sufficient
(if possible) to increase by its pressure the temperature of evaporation to
that of equilibrium. But the possibility of this will depend on the size of
the body, and the consequent attraction it has for its atmosphere ; for it is
clear that there must be a limit to the pressure which the atmosphere can
exert on the surface of the body ; and this limit must depend on the size of
the body. Up to a certain point, the thicker the atmosphere the greater
would be the pressure on the surface ; yet there must be a limit beyond
which the extension of the atmosphere would produce no effect, a limit beyond
o. R. 2



18 ON COMETARY PHENOMENA. [3 A

which the external air would be so distant, that the central body would not
exert sufficient coercive force to retain it, so that all excess would go off,
expanding into space. Hence the temperature at which evaporation will
continue on any body, must depend on the size of the body. The smaller the
body, the lower will be this temperature.

If, then, the body is so small that the atmosphere, when at its greatest,
cannot restrain evaporation until the temperature is equal to the tempera-
ture of equilibrium, then the body will go on evaporating, and the vapour go
on expanding into space until it is all evaporated, or, at any rate, until all
the softer materials, those which evaporate at a low temperature, are gone.
Thus we see that no body whose temperature of equilibrium remains fixed
that is to say, no body which moves round the sun in a circle no planet, in
fact, can remain for ever in a condition of permanent evaporation ; for in
time, no matter how long, it would lose all those materials on its surface
which would evaporate at a temperature below the temperature of equili-
brium. This, then, is the reason why there is not permanent evaporation
going on on the earth. The temperature of equilibrium may be taken
roughly at something like 50, whereas the temperature at which the most
volatile material (water) will boil is 212. If, however, the earth were to
approach the sun until its temperature of equilibrium rose to 300, then the
water would commence evaporating, until either the pressure of the atmo-
sphere of vapour was sufficient to stop further boiling, or else until it was
all gone*. Or if, on the other hand, the size of the earth were reduced so
that it was no longer able to retain an atmosphere whose pressure on its
surface was sufficient to prevent water boiling at 60 F., then the water
would go on boiling until it was all consumed. We see, then, that the
earth owes its stable condition to being so far away from the sun, and to
being of such size that it can retain an atmosphere, sufficient to prevent its
softest material from evaporating at a temperature below that of equilibrium,
and that in all bodies where this is not the case evaporation will be going
on. We may now see why comets should generally be in a state of evapora-
tion, even though they may not contain softer materials than water, and
may not approach nearer the sun than the distance of the earth. The fact
of their being so much smaller, prevents their retaining the same pressure of
atmosphere ; and so their materials evaporate at a lower temperature. The
eccentricity of their orbits is also essential to the explanation ; for if they
remained at a constant distance, they must eventually lose all the material
which would evaporate at that distance, and so become like the other
planets. This will be the case with periodic comets, those which, in spite of
the eccentricity of their orbits, return again and again ; for each time they

* It is true that water evaporates from its surface at a temperature much below 212; but an
atmosphere of steam would soon prevent this.



3 A] ON COMETARY PHENOMENA. 19

come near enough to the sun for the temperature of equilibrium to rise
above that of evaporation, they will lose some of their softer materials, until
these are all done ; and then, so far as evaporation is concerned, the comets
will behave as planets.

This may appear as though it were incompatible with the existence of
periodic comets. It is not so, however; it is only incompatible with the
permanence of periodic comets ; and it is an explanation of the facts :
(1) that whilst there are apparently a countless number of comets which do
not return, and, according to the laws of gravity, a countless number of these
must have been converted by the disturbances of the planets into periodic
comets, there are only a very few which are known to be periodic ; (2) that
the size of the periodic comets has been observed in many instances, if not
in all, to decrease ; (3) that there are many meteoric stones whose orbits are
similar to those of many of the periodic comets, and which do not show
cometic appearances, the assumption being that numberless comets have
been disturbed in their paths through space, and, instead of having been
sent back in a parabolic orbit, have, owing to a second disturbance by one of
our planets (generally Jupiter or Uranus) been attached to our system, and,
for a time, have appeared as periodic comets such as those of Halley, but
that they gradually lost their softer materials, becoming less and less, until
they finally ceased to be comets, and became meteoric stones.

The rate of evaporation on such a body as a comet, would obviously
increase as the comet approached the sun, and diminish as it receded ; but it
would not depend solely on the distance of the bodies from each other ; for
the materials of the comet would take time to heat, and consequently, as it
was approaching, part of the heat would go to warming the body of the
comet, and for any position the evaporation would be less than the sun would
cause if the comet were stationary. As it left the sun it would be the other
way ; that is, the evaporation would be more than the position warranted.
Thus the greatest rate of evaporation would not be exactly at the time when
the comet was nearest the sun, but some time after it had passed its peri-
helion. Now this lagging (as it may be called) of the sun's action on the
comet, is similar to, and consequently offers an explanation of, the lagging
which is observed in the display of comets. This will be seen from the
following quotation from Herschel :

" Their variations in apparent size, during the time they continue visible,
are no less remarkable than those of their velocity. Sometimes they make
their first appearance as faint and slow-moving objects with little or no tail ;
but by degrees accelerate, enlarge, and throw out from them this appendage,
which increases in length and brightness till (as always happens in such cases)
they approach the sun and are lost in his beams. After a time they again

22



20 ON COMETARY PHENOMENA. [3 A

emerge on the other side, receding from the sun with a velocity at first rapid,
but gradually decaying. It is for the most part after thus passing the sun
that they shine forth in all their splendour, and that their tails acquire their
greatest length and development, thus indicating plainly the action of the
sun's rays as the exciting cause of that extraordinary emanation."

The direct heat of the sun would only cause evaporation on that side of
the comet to which it was opposite ; and consequently the stream of vapour
would be emitted towards the sun, just as appears to be the case from obser-
vation ; the streams of vapour would first form an atmosphere round the
comet, which would increase until the extent was such that its attractive
force could no. longer prevent the outside being driven away by any force
there might be, and so forming a tail or train such force, for instance, as
would be exerted if the sun and vapour were both charged with electricity,
and acted on each other by induction.

Again, as the vapour proceeded outwards from the comet, it would rapidly
expand ; and this expansion would cause clouds to form by condensation,
which would travel outwards till they were again dispelled by the sun's rays ;
so that, on the side towards the sun, it is probable there might be one or
several shells of cloud at certain distances from the central mass. These,
under the action of the sun's rays, would be illuminated, and afford that
striking appearance of several bands which is so often seen.

The effect of any repulsive or attractive force in the sun, acting on the
vapour of the comet, but not on the central mass, would cause a train or



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