Yet, even as Herapath’s work was being rejected developments were afoot that would change the picture of the atom and solve the problem of collisions without invoking caloric.  In 1802 Dalton had imagined atmospheres of caloric surrounding each particle.  With the development of the electro-chemical theory of matter in the 1820’s and ‘30’s, the atmosphere of caloric was replaced by an atmosphere of electricity.  The fundamental particles of matter were still perfectly hard, but they were protected from actual contact in a collision by their electrical quality.

Herapath’s hypothesis would be revived in the 1840’s sans problem of hardness.  Here for example is Joule writing in 1848:

…since the hypothesis of Herapath, in which it is assumed that the particles of a gas are constantly flying about in every direction with great velocity, the pressure of the gas owing to the impact of the particles against any surface presented to them, is somewhat simple, I shall employ it…

What had only 30 years before been stymied by an impassable metaphysical barrier was now a simple hypothesis.  In this fashion, we find Herapath’s work built upon by such master physicists as Joule, Maxwell, Kelvin, and Clausius.  The kinetic theory had been rescued by the new electro-chemical atom…a complex, whirling, and clanking beast most fit for the new industrial age.

In a text on the kinetic theory, written in 1872, Maxwell pictures atoms colliding.  He begins by observing that the molecules of a gas are not acted upon by any sensible force for the greater part of the time, and so move in straight lines with uniform velocities.  When they move close enough to one another, the molecules act on one another with a force that grows as they approach and then weakens as they move apart.  He then adds: “the free motion of a molecule takes up much more time than that occupied by an encounter.”

And so, Maxwell sets aside the conflict between Herapath and the reasoning of Newton et al.  The bulk of the time a particle is effectively free, and not in collision.  Collisions are mediated by some force, unspecified and unproblematic.  A collision, or an encounter, was simply strong repulsive forces acting as particles approached one another and receded…at least that is all a collision had become.  It had been something very different when the young Herapath had laid out his argument.