Encyclopaedia Britannica > Volume 3, Athens-BOI

ATOM
47
velocity equal to that which they had before, they will
carry their energy away with them into the ultramundane
regions. But if this be the case, then the corpuscules
rebounding from the body in any given direction will be
both in number and in velocity exactly equivalent to those
which are prevented from proceeding in that direction by
being deflected by the body, and it may be shown that this
will be the case whatever be the shape of the body, and
however many bodies may be present in the field. Thus, the
rebounding corpuscules exactly make up for those which
are deflected by the body, and there will be no excess of
the impacts on any other body in one direction or another.
The explanation of gravitation, therefore, falls to the
ground if the corpuscules are like perfectly elastic spheres,
and rebound with a velocity of separation equal to that of
approach. If, on the other hand, they rebound with a
smaller velocity, the effect of attraction between the bodies
will no doubt be produced, but then we have to find what
becomes of the energy which the molecules have brought
with them but have not carried away.
If any appreciable fraction of this energy is communicated
to the body in the form of heat, the amount of heat so
generated would in a few seconds raise it, and in like
manner the whole material universe, to a white heat.
It has been suggested by Sir W. Thomson that the
corpuscules may be so constructed so to carry off their
energy with them, provided that part of their kinetic energy
is transformed, during impact, from energy of translation
to energy of rotation or vibration. For this purpose the
corpuscules must be material systems, not mere points.
Thomson suggests that they are vortex atoms, which are
set into a state of vibration at impact, and go off with a
smaller velocity of translation, but in a state of violent
vibration. He has also suggested the possibility of the
vortex corpuscule regaining its swiftness and losing part
of its vibratory agitation by communion with its kindred
corpuscules in infinite space.
We have devoted more space to this theory than it seems
to deserve, because it is ingenious, and because it is the
only theory of the cause of gravitation which has been so
far developed as to be capable of being attacked and
defended. It does not appear to us that it can account for
the temperature of bodies remaining moderate while their
atoms are exposed to the bombardment. The temperature
of bodies must tend to approach that at which the average
kinetic energy of a molecule of the body would be equal to
the average kinetic energy of an ultramundane corpuscule.
Now, suppose a plane surface to exist which stops all the
corpuscules. The pressure on this plane will be p = NMw2
where M is the mass of a corpuscule, N the number in
unit of volume, and u its velocity normal to the plane.
Now, we know that the very greatest pressure existing in
the universe must be much less than the pressure p, which
would be exerted against a body which stops all the
corpuscules. We are also tolerably certain that N, the
number of corpuscules which are at any one time within
unit of volume, is small compared with the value of N for
the molecules of ordinary bodies. Hence, Mw2 must be
enormous compared with the corresponding quantity for
ordinary bodies, and it follows that the impact of the
corpuscules would raise all bodies to an enormous tempera¬
ture. We may also observe that according to this theory
the habitable universe, which we are accustomed to regard
as the scene of a magnificent illustration of the conservation
of energy as the fundamental principle of all nature, is in
reality maintained in working order only by an enormous
expenditure of external power, which would be nothing
less than ruinous if the supply were drawn from anywhere
else than from the infinitude of space, and which, if the
contrivances of the most eminent mathematicians should be
found in any respect defective, might at any moment tear
the whole universe atom from atom.
We must now leave these speculations about the nature
of molecules and the cause of gravitation, and contemplate
the material universe as made up of molecules. Every
molecule, so far as we know, belongs to one of a definite
number of species. The list of chemical elements may be
taken as representing the known species which have been
examined in the laboratories. Several of these have been
discovered by means of the spectroscope, and more may
yet remain to be discovered in the same way. The spec¬
troscope has also been applied to analyse the light of the
sun, the brighter stars, and some of the nebulas and comets,
and has shown that the character of the light emitted by
these bodies is similar in some cases to that emitted by
terrestrial molecules, and in others to light from which the
molecules have absorbed certain rays. In this way a
considerable number of coincidences have been traced
between the systems of lines belonging to particular
terrestrial substances and corresponding lines in the spectra
of the heavenly bodies.
The value of the evidence furnished by such coincidences
may be estimated by considering the degree of accuracy
with which one such coincidence may be observed. The
interval between the two lines which form Fraunhofer’s
line D is about the five hundredth part of the interval
between B and Gr on Kirchhoff’s scale. A discordance
between the positions of two lines amounting to the tenth
part of this interval, that is to say, the five thousandth
part of the length of the bright part of the spectrum,
would be very perceptible in a spectroscope of moderate
power. We may define the power of the spectroscope to
be the number of times which the smallest measurable
interval is contained in the length of the visible spectrum.
Let us denote this by p. In the case we have supposed p
will be about 5000.
If the spectrum of the sun contains n lines of a certain
degree of intensity, the probability that any one line of the
spectrum of a gas will coincide with one of these n lines is
1 -('l-A = - +&c\
\ pj p\ 2 p J’
and when p is large compared with n, this becomes
nearly -. If there are r lines in the spectrum of the
gas, the probability that each and every one shall coincide
wixn a nne in tne
Hence, in the case of a gas whose spectrum contains several
lines, we have to compare the results of two hypotheses.
If a large amount of the gas exists in the sun, we have the
strongest reason for expecting to find all the r lines in the
solar spectrum. If it does not exist, the probability that
r lines out of the n observed lines shall coincide with the
lines of the gas is exceedingly small. If, then, we find all
the r lines in their proper places in the solar spectrum, we
have very strong grounds for believing that the gas exists
in the sun. The probability that the gas exists in the sun
is greatly strengthened if the character of the lines as to
relative intensity and breadth is found to correspond in
the two spectra.
The absence of one or more lines of the gas in the solar
spectrum tends of course to weaken the probability, but
the amount to be deducted from the probability must
depend on what we know of the variation in the relative
intensity of the lines when the temperature and the pres¬
sure of the gas are made to vary.
Coincidences observed, in the case of several terrestrial
substances, with several systems of lines in the spectra of
the heavenly bodies, tend to increase the evidence for the