1822.) C.’s Reply to D 905 
the atoms or particles, and is proportional to their individual 
momentum ;” and that the temperature of bodies is equal when 
the velocities of the particle are inversely proportional to their 
magnitude ; I concluded that “‘the greater atoms having less 
velocity than’the smaller will never overtake them.” Upon this 
D. observes: “ That is not universally the case. In consequence 
of the mutual action of the particles, they move both in their 
goings and returnings swifter at some parts of their paths than 
at others. Generally speaking, in the exterior particles, which 
are those of the two bodies that come in contact, their velocities 
are the swiftest immediately before and after the collision ; and 
the slowest immediately preceding and following the exterior 
extremity of their path. Hence, therefore, the greater particles 
may often move much swifter than the less ; and, consequently, 
may frequently overtake and strike them.” Thus it is assumed, 
that the particles have paths, to which there are extremities or 
limits, at which they return, and near to which they move so 
much slower than at other parts of their paths, that the respective 
velocities of the particles at the time of their collision, may be 
directly the opposite of that of the mean motion of the bodies to 
which they belong. But in the preceding page, in relation to 
the very same particles of the same bodies, he says, ‘* nor can 
it” (the particles) “return to its own body, because the collision 
did not give it an inward, but merely diminished its outward 
motion.” Now this assumption that the particle will not return 
to its own body after the first collision, till it comes into collision 
with, and receives an inward motion from, another particle of the 
second body, is directly contrary to the other supposition that 
the particles have limited paths, at the extremities of which they 
return of themselves. 
He proceeds, ‘ Now the outward particle which” (the particle 
of the body A) ‘ next strikes, must evidently meet it with the 
mean motion B b-of the particles to which it belongs.” But his 
other assumption is, that the exterior particle of bodies near the 
extremities of their paths (and there alone solid bodies could 
come into contact if they had the supposed motions) “ generally 
speaking,” move slower than the mean motion of the body. Nor 
can he with propriety found the supposed mean motion upon any 
contemplation of mine. I have not supposed any such second 
collision at all; nor is it probable (if it be reasonable to use such 
a term in relation to such a theory) that upon the supposed facts 
there would be a second collision. 
However contradictory each of these suppositions is to the 
other, it is equally opposed to facts. For if it were true that the 
exterior particles ‘ which are those of the two bodies which 
come in contact,” had, generally speaking, a slower motion at 
the extremity of their paths, the communication of motion (that 
is, according to Mr. H.’s theory, the communication’ of temper- 
ature) from body to body, would not depend so much upon the 
