FREE AND PERFECTLY ELASTIC MOLECULES IN A STATE OF MOTION. 17 
We have deduced, in reference to media, that when they are in equilibrium of 
pressure and vis viva they have, in equal volumes, the same amount of vis viva. 
(3.) It is considered probable that the quantity of heat contained in equal volumes 
of all gases in equilibrium of pressure and temperature is proportional to their absolute 
temperature; and, in general, that equal increments of heat cause equal increments of 
temperature or expansion. 
In all media in equilibrium of pressure and vis viva the same increment of vis viva 
causes the same expansive effect. (See also § 6.) 
(4.) Gases, however different in specific gravity, when placed in connection diffuse 
themselves equally through each other; and according to the elegant induction of 
Dr. Graham (‘ Edin. Trans.,’ 1831) the velocity of diffusion is inversely as the square 
root of their specific gravity. 
This is very exactly responded to by the hypothesis. Dalton’s law of diffusion 
belongs to media as a necessary and the most obvious consequence of the constitution 
assigned to them (VI.) Dr. Graham’s law of diffusive velocity or volume applies 
also to media, because if placed in similar circumstances such an effect must depend 
on the molecular velocity, which, singularly enough, bears exactly the like ratio to 
their specific gravity (X.). 
§ 15. In the first point of analogy it was stated the atomic weight of a gas 
corresponded with its specific gravity, but with the reservation that the combining 
proportions are simple multiples or quotients of the same. This appears to me to be 
the fair statement of the remarkable connection that is always found between the 
combining volume and combining weight. It seems impossible that the fact of a 
volume of every gas containing the same number of molecules can ever be inductively 
established, but all analogy leads us to this conclusion. One volume of oxygen 
combines with two volumes of hydrogen to form two volumes of aqueous vapour. If 
we inferred from this that one molecule of oxygen combined with two molecules of 
hydrogen to form one of steam, we must admit that the molecule of steam occupies 
double the volume of a molecule of hydrogen or oxygen. If it is admitted, on the 
other hand, that the oxygen molecule is capable of disintegration, and that half a 
molecule combines with one of hydrogen to form one of steam, the bulk of the three 
molecules are equal. This last is the view that is responded to by the hypothetical 
media. The objection to it is plausible from the natural repugnance to the idea of 
dividing what has been considered as an ultimate element into parts, and of supposing 
it possible that an element should have a strong affinity to itself—for this point is 
also involved. Half molecules of oxygen must have a powerful attraction to each 
other as they never appear separate. It is the same with all the other simple gases 
and vapours that combine in half or other fractional volumes. We have an analogous 
example among the hydrocarburets. Methyl is half a molecule of olefiant gas, and 
enters into distinct combinations similar in ever}'' respect to those of olefiant gas, but it 
never appears in a separate form. Olefiant gas enters into combination with itself in 
MDCCCX'HT.—A. D 
