Encyclopaedia Britannica > Volume 3, Athens-BOI

ATOM
48
doctrine that terrestrial substances exist in the heavenly
bodies, while the discovery of particular lines in a celestial
spectrum which do not coincide with any line in a terres¬
trial spectrum does not much weaken the general argument,
but rather indicates either that a substance exists in the
heavenly body not yet detected by chemists on earth, or that
the temperature of the heavenly body is such that some
substance, undecomposable by our methods, is there split
up into components unknown to us in their separate state.
We are thus led to believe that in widely-separated parts
of the visible universe molecules exist of various kinds, the
molecules of each kind having their various periods of
vibration either identical, or so nearly identical that our
spectroscopes cannot distinguish them. We might argue
from this that these molecules are alike in all other
respects, as, for instance, in mass. But it is sufficient for
our present purpose to observe that the same kind of
molecule, say that of hydrogen, has the same set of periods
of vibration, whether we procure the hydrogen from water,
from coal, or from meteoric iron, and that light, having the
same set of periods of vibration, comes to us from the sun,
from Sirius, and from Arcturus.
The same kind of reasoning which led us to believe that
hydrogen exists in the sun and stars, also leads us to believe
that the molecules of hydrogen in all these bodies had a
common origin. For a material system capable of vibra¬
tion may have for its periods of vibration any set of
values whatever. The probability, therefore, that two mate¬
rial systems, quite independent of each other, shall have,
to the degree of accuracy of modern spectroscopic measure¬
ments, the same set of periods of vibration, is so very small
that we are forced to believe that the two systems are not
independent of each other. When, instead of two such
systems, we have innumerable multitudes all having the
same set of periods, the argument is immensely strength¬
ened.
Admitting, then, that there is a real relation between
any two molecules of hydrogen, let us consider what this
relation may be.
We may conceive of a mutual action between one body
and another tending to assimilate them. Two clocks, for
instance, will keep time with each other if connected by a
wooden rod, though they have different rates if they were dis¬
connected. But even if the properties of a molecule were as
capable of modification as those of a clock, there is no physical
connection of a sufficient kind between Sirius and Arcturus.
There are also methods by which a large number of
bodies differing from each other may be sorted into sets, so
that those in each set more or less resemble each other.
In the manufacture of small shot this is done by making
the shot roll down an inclined plane. The largest specimens
acquire the greatest velocities, and are projected farther
than the smaller ones. In this way the various pellets,
which differ both in size and in roundness, are sorted into
different kinds, those belonging to each kind being nearly
of the same size, and those which are not tolerably spherical
being rejected altogether.
If the molecules were originally as various as these leaden
pellets, and were afterwards sorted into kinds, we should
have to account for the disappearance of all the molecules
which did not fall under one of the very limited number
of kinds known to us; and to get rid of a number of
indestructible bodies, exceeding by far the number of the
molecules of all the recognised kinds, would be one of the
severest labours ever proposed to a cosmogonist.
It is well known that living beings may be grouped into
a certain number of species, defined with more or less preci¬
sion, and that it is difficult or impossible to find a series of
individuals forming the links of a continuous chain between
one species and another. In the case of living beings,
however, the generation of individuals is always going
on, each individual differing more or less from its
parents. Each individual during its whole life is under¬
going modification, and it either survives and propa¬
gates its species, or dies early, accordingly as it is more
or less adapted to the circumstances of its environment.
Hence, it has been found possible to frame a theory of
the distribution of organisms into species by means of
generation, variation, and discriminative destruction. But
a theory of evolution of this kind cannot be applied to the
case of molecules, for the individual molecules neither are
born nor die, they have neither parents nor offspring, and
so far from being modified by their environment, we find
that two molecules of the same kind, say of hydrogen, have
the same properties, though one has been compounded
with carbon and buried in the earth as coal for untold
ages, while the other has been “ occluded” in the iron
of a meteorite, and after unknown wanderings in the
heavens has at last fallen into the hands of some terrestrial
chemist.
The process by which the molecules become distributed
into distinct species is not one of which we know any
instances going on at present, or of which we have as yet
been able to form any mental representation. If we suppose
that the molecules known to us are built up each of some
moderate number of atoms, these atoms being all of them
exactly alike, then we may attribute the limited number of
molecular species to the limited number of ways in which
the primitive atoms may be combined so as to form a
permanent system.
But though this hypothesis gets rid of the difficulty of
accounting for the independent origin of different species
of molecules, it merely transfers the difficulty from the
known molecules to the primitive atoms. How did the
atoms come to be all alike in those properties which are in
themselves capable of assuming any value 1
If we adopt the theory of Boscovich, and assert that the
primitive atom is a mere centre of force, having a certain
definite mass, we may get over the difficulty about the
equality of the mass of all atoms by laying it down as a
doctrine which cannot be disproved by experiment, that
mass is not a quantity capable of continuous increase or
diminution, but that it is in its own nature discontinuous,
like number, the atom being the unit, and all masses being
multiples of that unit. We have no evidence that it is
possible for the ratio of two masses to be an incommensur¬
able quantity, for the incommensurable quantities in
geometry are supposed to be traced out in a continuous
medium. If matter is atomic, and therefore discontinuous,
it is unfitted for the construction of perfect geometrical
models, but in other respects it may fulfil its functions.
But even if we adopt a theory which makes the equality
of the mass of different atoms a result depending on the
nature of mass rather than on any quantitative adjustment,
the correspondence of the periods of vibration of actual
molecules is a fact of a different order.
We know that radiations exist having periods of vibration
of every value between those corresponding to the limits
of the visible spectrum,, and probably far beyond these
limits on both sides. The most powerful spectroscope can
detect no gap or discontinuity in the spectrum of the light
emitted by incandescent lime.
The period of vibration of a luminous particle is therefore
a quantity which in itself is capable of assuming any one
of a series of values, which, if not mathematically con¬
tinuous, is such that consecutive observed values differ
from each other by less than the ten thousandth part of
either. There is, therefore, nothing in the nature of time
itself to prevent the period of vibration of a molecule from
assuming any one of many thousand different observable