460 On the two great powers, (Szpr. 
It is certainly true, that when a body contracts on a reduction, or ex- 
pands on arise of temperature, in the one case the particles approach, 
and in the other recede, from each other. This approximation, and se- 
paration, is a gradual and regular motion. Thus, if two particles are 
kept at a certain distance from each other, by any force (whether of 
heat or not) on the removal of that force the particles must have mo- 
tion, if they approach; but when they arrive the distance at which 
they are to remain, this motion ceases, and is no proof of vibratory 
motions in the atoms, nor can it give rise to them. When particles 
approach, they are put into gradual motion, by the force of attraction ; 
and they will not separate, until a superior force urges them in a dif- 
ferent direction, 
Since the particles of matter have space between them, and since they 
exert great attractions for each other, the force, which keeps them 
asunder, must be equal to their attraction. If this force isa vibration of 
the particles, it cannot be permanent. No motion can be lasting, when 
opposed by any force, however small, unless it is preserved by an equal 
force. But the vibration of atoms would be opposed by a very power- 
ful force, their mutual attraction ; which would urge them into absolute 
contact ; when any vibration must cease, from their impact against each 
other. It cannot be said, that their motion is kept up by that of 
neighbouring bodies, for the vibration of all particles in nature would 
very soon cease for the same reason. 
The expansion, which a hot body produces, in one of a lower tem- 
perature, arises from the divellent power becoming superior to their 
attraction, and producing a slow and progressive separation of the par- 
ticles of the latter, which power, as above shewn, cannot depend upon 
a vibratory motion, for any such motion must soon cease. And even 
could such motion last, it would not be increased by superior vibrations 
in another body, but lessened. If two vibrating bodies are brought into 
contact, their vibrations cease directly, from the one body being a 
mechanical obstacle to any motions in the other. But, if the motion 
in the one is greater, it will still more check any motion in the other, 
not only from the obstacle arising from its contact, but also from its 
increased impact, unless it be supposed, that the particles of the two 
bodies happen to be moving in the same direction, at the instant of 
their contact. This, which would involve the idea, that all particles 
in nature are always oscillating in the same direction, at the same 
moment of time, is moreover contrary toa supposition in the above 
theory, that bodies of different temperatures vibrate with different 
velocities, from which their atoms would soon move in different direc- 
tions at the same time. 
