106 On the Cause of Heat. 
consist of corpuscular motions is, that mechanical attritiow ge- 
nerates it. Surely then a motion produced by mechanical means, 
and which produces mechanical effects, may be estimated on me- 
chanical principles, 
In the case cited above, the power of reciprocal communica- 
tion of heat in two fluids, .is shown to be inconsistent with the 
views of this ingenious theorist. If we compare the same power 
in solids, the result will be equally objectionable. ‘Thus the 
heating power of glass being 443, that of an equal bulk of lead 
will be 487, though so many times heavier ;. and if equal weights 
be compared, the effect of the glass will be four times greater 
than that of the lead. If it be said, that the movements of the 
denser matter are made in less space, and therefore require less 
motion, I answer, that if they be made with equal velocity, they 
must go through equal space in the same time, their alternations 
being more frequent. And if they be not made with the same 
velocity, they could not communicate to matter of a lighter kind 
a heat equally great; since, agreeably to experience, no supe- 
riority of weight will enable a body, acting directly on another, 
to produce in it a motion quicker than its own. Consistently 
with this doctrine, the particles of an aériform fluid, when they 
oppose a mechanical resistance, do it by aid of a certain move- 
ment, which causes them effectively to occupy a greater space 
than when at rest. Itis true, a body by moving backwards and 
forwards may keep off other bodies from the space in which it 
moves. Thus, let a weight be partially counterbalanced by means 
of a scale beam, so that if left to itself it would descend gently. 
Place exactly under it another equally solid mass, on which the 
weight would fall. If between the two bedies thus situated a 
third be caused to undergo an alternate motion, it may keep the 
upper weight from descending, provided the force with which 
the latter descends be no greater than that of the movement in 
the interposed mass, and the latter acts with such celerity, that 
between each stroke the time be too-small for the weight to 
moye any sensible distance. Here then we have a case analogous 
to that supposed, in which the alternate inovements or vibrations 
of matter enable it to preserve to itself a greater space 1n oppo- 
sition to a force impressed; and it must be evident that lengthen- 
ing or shortening the extent of the vibrations of the interposed 
body, provided they are made in the same time, will increase or 
diminish the space apparently occupied by it, as the volume of 
substances is affected by an increase or reduction of heat. It 
ought however to be recollected that in the case we have imagined, 
there is a constant expenditure of momentum to compensate for 
that generated in the weight by gravity, during each vibration. 
lu the vibrations conceived to constitute heat, there is no gene- 
rating 
