To Dr. HUGGINS, F.R.S.
Ardhallow, Dunoon, Oct. 13/68.
MY DEAR SIR—I sympathise with
you in your great
sorrow. Though
my own mother was only eight years with me, and my father
became
my companion in all things, I felt her loss for many years, and can in
some degree appreciate your happiness in having so long and
so complete fellowship with your mother. I have little
fear,
however, that the nearness to the other world which you must feel will
in any way unfit you for the work on which you have been engaged,
for the higher powers of the intellect are strengthened by the exercise
of the nobler emotions.
· · · · ·
Your
identification of the
spectrum of comet
11 with that of carbon is very wonderful. The dynamical state
of
comets’ tails is most perplexing, but the chemistry and activity of
their
heads leads to new questions. With respect to the
transparency
of a heavenly body, I think it indicates scattered condition rather
than
gaseity. A cloud of large blocks of stone is much more
transparent
than air of the sense average density. Such blocks in a nebula
would
never be themselves seen, but perhaps if they were often to encounter
each
other, the results of the collision would be incandescent
gases,
and might be the only visible part of the nebula.
. . . Any
opinion as to the
form in which the
energy of gravitation exists in space is of great importance,
and
whoever can make his opinion probable will have made an enormous stride
in physical speculation. The apparent universality of
gravitation, and the equality of its effects on matter of all kinds are
most remarkable facts, hitherto without exception;
but
they are purely experimental facts, liable to be corrected by a single
observed exception. We cannot conceive of matter with negative
inertia
or mass; but we see no way of accounting for the propor-tionality
of gravitation to mass by any legitimate method of demonstration. If we
can see the tails of comets fly off in the direction
opposed
to the sun with an accelerated velocity, and if we believe these tails
to be matter and not optical illusions or mere tracks of
vibrating
disturbance, then we must admit a force in that direction, and we
may establish that it is caused by the sun if it always depends upon
his
position and distance. I there-fore admit that the
proposition
that the sun repels comets’ tails is capable of proof; but whether he
does
so by his ordinary attrac-tive power being changed into
repulsion
by a change of state of the matter of the tail is another
question.
Now, it seems ascertained by simple observations with telescopes that
the
coma is foraged by successive explosions out of the
nucleus,
mostly on the side of the sun, and that the formation of the tail
depends
on the coma, though the substance is invisible in the state
of passing from the coma to the tail. Then, by your observations,
the
nucleus and coma have light of their own, probably due to carbon in
some
gaseous form; but the tail’s light being polarised in the plane
of
the sun is due to him. Hence the head is fire and the tail smoke. The
head
obeys
gravitation, which is exerted on it with precisely the same intensity
as on all other known matter, solid or gaseous.
The tail appears to be acted on in a contrary
way. If the comet consisted of a mixture of gravitating and
levitating
matter, and is analysed by the sun, then before the emission of the
tail
the acceleration due to gravitation should be less than on
a planet at the same distance; the more com-plete the discharge of tail
the greater the intensity of gravitation on the remaining
head.
N.B.—To understand the dynamics of the
tail, the motion
in space of particular portions of it must be studied.
