ON THE MOLECULAR MOBILITY OE GASES. 
171 
■whatever it may be. I cannot account otherwise for the slight predominance which the 
lighter and faster gas appears always to acquire in diffusing through the porous septum. 
Speculative Ideas respecting the Constitution of Matter. 
It is conceivable that the various kinds of matter, now recognised as different ele¬ 
mentary substances, may possess one and the same ultimate or atomic molecule existing 
in different conditions of movement. The essential unity of matter is an hypothesis in 
harmony with the equal action of gravity upon all bodies. We know the anxiety with 
which this point was investigated by Newton, and the care he took to ascertain that 
every kind of substance, “metals, stones, woods, grain, salts, animal substances,” etc., 
are similarly accelerated in falling, and are therefore equally heavy. 
In the condition of gas, matter is deprived of numerous and varying properties with 
which it appears invested when in the form of a liquid or solid. The gas exhibits only a 
few grand and simple features. These again may all be dependent upon atomic and 
molecular mobility. Let us imagine one kind of substance only to exist, ponderable 
matter; and further, that matter is divisible .into ultimate atoms, uniform in size and 
weight. We shall have one substance and a common atom. With the atom at rest the 
uniformity of matter would be perfect. But the atom possesses always more or less 
motion, due, it must be assumed, to a primordial impulse. This motion gives rise to 
volume. The more rapid the movement the greater the space occupied by the atom, 
somewhat as the orbit of a planet widens with the degree of projectile velocity. Matter 
is thus made to differ only in being lighter or denser matter. The specific motion of an 
atom being inalienable, light matter is no longer convertible into heavy matter. In 
short, matter of different density forms different substances,—different inconvertible 
elements, as they have been considered. 
What has already been said is not meant to apply to the gaseous volumes which we 
have occasion to measure and practically deal with, but to a lower order of molecules or 
atoms. The combining atoms hitherto spoken of are therefore not the molecules of 
which the movement is sensibly affected by heat, with gaseous expansion as the result. 
The gaseous molecule must itself be viewed as composed of a group or system of the 
preceding inferior atoms, following as a unit laws similar to those which regulate its 
constituent atoms. We have indeed carried one step backward, and applied to the lower 
order of atoms, ideas suggested by the gaseous molecule, as views derived from the solar 
system are extended to the subordinate system of a planet and its satellites. The 
advance of science may further require an indefinite repetition of such steps of molecular 
division. The gaseous molecule is then a reproduction of the inferior atom on a higher 
scale. The molecule or system is reached which is affected by heat, the diffusive mole¬ 
cule of which the movement is the subject of observation and measurement. The dif¬ 
fusive molecules are also to be supposed uniform in weight, but to vary in velocity of 
movement, in correspondence with their constituent atoms. Accordingly the molecular 
volumes of different elementary substances have the same relation to each other as the 
subordinate atomic volumes of the same substances. 
But further, these more and less mobile or light and heavy forms of matter have a 
singular relation connected with equality of volume. Equal volumes of two of them 
. can coalesce together, unite their movement, and form a new atomic group, retaining 
the whole, the half, or some simple proportion of the original movement and consequent 
volume. This is chemical combination. It is directly an affair of volume, and only 
indirectly connected with weight. Combining weights are different, because the densities, 
atomic and molecular, are different. The volume of combination is uniform, but the 
fluids measured vary in density. This fixed combining measure—the metron of simple 
substances—weighs 1 for hydrogen, 1G for oxygen, and so on with the other “elements.” 
To the preceding statements respecting atomic and molecular mobility, it remains to 
be added that the hypothesis admits of another expression. As in the theory of light 
we have the alternative hypotheses of emission and undulation, so in molecular mobility 
the motion may be assumed to reside either in separate atoms and molecules, or in a 
fluid medium caused to undulate. A. special rate of vibration or pulsation originally 
imparted to a portion of the fluid medium enlivens that portion of matter with an in¬ 
dividual existence, and constitutes it a distinct substance or element. 
