28 
MR, J. J. WATERSTOH OR THE PHYSICS OF MEDIA COMPOSED OF 
we know, exists in nature. If molecular vis viva is heat, the molecules of solids must 
be in motion as well as those of gases; and the nature of the motion must be such as 
to permit the equilibrium of vis viva to be established between them respectively. 
They must also be perfectly elastic ; but a surface composed of them cannot be 
assumed as perfectly rigid. 
The original hypothesis in respect to gases involves the necessity of making certain 
assumptions respecting the physical condition of the surfaces upon which media are 
supposed to act. 
(1.) That they are composed of molecules in a state of vibratory motion which results 
from the struggle that their vis insita makes with the attractive and repulsive forces 
of aggregation. 
(2.) The nature of these vibrations and forces may remain undefined, further than 
that the vis insita proper to a molecule is alternately destroyed at the extremities, 
and reproduced in the middle of each vibration when it and the surrounding 
molecules are in equilibrium of vis viva. 
(3.) The impact of the molecules of a medium on the molecules of the solid surface 
is that of perfectly elastic bodies, and enables the equilibrium of vis viva to be estab¬ 
lished between them. 
This equilibrium must be effected by a continual interchange of vis viva, the 
molecules of the solid giving to the molecules of the medium and vice versa. 
In the case of a heavy molecular plane supported by the elasticity of the medium, as 
detailed in § 2 and § 16, the impacts that take place on the lower surface, establish 
both the molecular vis viva equilibrium, and also the statical equilibrium of the heavy 
plane. 
Does this new condition of surface upon which the medium acts, make any change 
in the relation ng/2v = A, that was shown to subsist between the impinging velocity of 
succession, and weight supported when the surface was assumed as perfectly rigid ? 
Then, the molecules of the medium encountered at each impact the whole vis insita of 
the heavy plane and communicated directly to its centre of gravity a certain infini¬ 
tesimal velocity. Now, it strikes a vibrating molecule which afterwards communicates 
a certain effect or infinitesimal velocity on the centre of gravity of the molecular plane. 
Let us endeavour to gain a clear idea of the numerical relation between the effect 
and the cause that produces it, viz., the impinging force of the free molecule of the 
medium. 
The centre of gravity of the whole molecular plane being at rest while the centre of 
gravity of each of its molecules is in a state of intense vibration, it is evident that 
the track or orbit described by the centre of gravity of one of its molecules must be' 
exactly imitated but on an infinitely reduced scale and in a reverse direction by the 
motion of the centre of gravity of the remaining molecules of the plane. The action 
and reaction of the molecular forces are equal. At any point of the orbit of one of the 
lower molecules of the plane let a free molecule of the medium impinge. If they are 
