52 Mr. J. S. Stuart Glennie on the 
pressure from molecules of the size to give by their vibrations 
the sensations of light and heat. 
It would seem that this conception of the “ ether” leads to 
the explanation of more than one difficulty in the egtablished 
theory ; but nothing can, of course, be advanced on such a point 
except as the result of analysis. | 
It will be understood that I thus speak of the conception of 
atoms, as above defined, as a mode of conceiving the “ther” of 
the undulatory theory, only in order to make clear the applica- 
tion to that theory of the fundamental conception of the general 
theory here proposed; and that this in nowise contradicts what 
has been above said as to the seeming needlessness of hypotheses 
of special fluids, or ethers, acting on, or through, matter. 
28. A general chemical theory is made up of two—a theory 
of the constitution, and a theory of the combination, of bodies. 
As to the constitution of bodies, the principal views here offered 
are :—Bodies are conceived as states of dynamic molecular equi- 
librium, that is, as states of molecular motion in which, while 
there is no decomposition, the intensity of motion is at all points 
equal. Hence, their differences are conceived as resulting from 
different conditions of molecular motion; and thus specific heat 
becomes one of the chief exponents of the nature of a body. 
The distinguishing mechanical characteristic of the gaseous, 
fluid, and solid, states of matter is degree of tangential resistance. 
In an absolutely perfect gas the molecules would be of equal 
mass and at equal distances: hence perfect equality of resist- 
ance in all directions. But let there be inequality either m the 
masses or distances of the molecular centres of pressures, it is 
evident, from the same reasoning as that above applied to the 
mechanical explanation of gravity, that there would be a cohesive 
force developed, the consequence of which would be inequality 
of resistance. Fluids, therefore, as opposed to solids, are con- 
ceived as bodies the molecules of which are of sensibly equal 
masses and at equal distances; and gases, as opposed to liquids, 
as bodies in which the distances of the molecules, though equal, 
are greater than in liquids. Hence the greater amplitude of their 
motion, or specific heat. 
The consideration of this theory of bodies with reference to 
that of Kronig* and Clausius+, cannot be at present entered upon. 
* “ Grundziige emer Theorie der Gase,” Poge. Ann. xcix. 315. 
+ “ Ueber die bewegende Kraft der Warme,”’ Poge. Ann. Ixxix. 394; and 
“ Ueber die Art der Bewegung welche wir Warme nennen,”’ ibid. c. 353. See 
also Prof. Maxwell’s “Illustrations of the Dynamical Theory of Gases,” Phil. 
Mag., January and July 1860. Dr. Tyndall’s discoveries and researches 
“On the Transmission of Heat of different qualities through Gases of 
different kinds ”’ (Proceedings of the Royal Institution, June 10, 1859), 
are of the greatest importance in such a dynamical theory of the constitu- 
tion of bodies, 
