Experimental Contact with Molecules

Abstract

Perrin’s experiments on Brownian motion left little room for reasonable doubt about the existence of molecules. They provided independent support for the statistical fluctuations that had been postulated by proponents of the kinetic theory and they gave direct support to two basic assumptions of that theory, the randomness of the molecular motions via the motions of the Brownian particles that they jostled, and the equipartition of energy for motions of translation and rotation. Three methods for calculating Avogadro’s number were in agreement with each other and with estimates acquired by other means. The kinetic theory was not confirmed in an unqualified way. It became increasingly clear that violations of the equipartition of energy constituted a problem that could not be solved without a drastic modification in the fundamentals of the theory. The challenge around the time of Perrin’s experiments was to clarify what the kinetic theory got right and what it got wrong. Perrin was able to show that it got things right to an extent that it became unreasonable to doubt the existence of molecules.