it was off. Here, then, is an absolutely direct proof that the ion must he endowed with a kinetic energy of agitation which is sufficient to push it up to the surface of the drop against the electrostatic repulsion of the charge already existing on the drop, and when we remember that an ion is nothing but a molecule containing an unneutralized electrical charge, it will be clear that we have here direct proof that the molecules of the gas are endowed with motion.
Furthermore, it is easy to obtain the energy of this motion, for, if we load up the drop with more and more positive charges, the push which it will exert on positive ions within the gas must become greater and greater, and hence the frequency with which positive ions will be captured from the gas should become less and less. Now this is exactly what was observed to be the case, and, indeed, in one instance, a relatively heavily charged drop was watched for four hours, during which time it succeeded in picking up but one single ion of its own sign while the field was off, although it was continually picking up ions of the opposite sign. Its charge was during all this time maintained at about the same value by forcing ions of its own kind upon it when the field was on. We had then here a charged drop which exerted just enough repulsion upon the positive ions of the gas to overcome their kinetic energy of agitation when they shot toward it. By knowing the size of the drop and the charge which it carried, it was easy to compute from these two quantities just what this kinetic energy of agitation had to be in this case. It came out within a few per cent, of the value of the kinetic energy of agitation of the molecules as given by the kinetic theory.
But even this evidence is not sufficiently direct to convince a skeptic untrained to follow the computation, simple though it be. Hence a proof was sought which involved no knowledge whatever of either mathematical or physical theory. Fortunately the trail had already been blazed and nothing had to be done but to clear out some of the remaining underbrush which obscured it. It had been discovered as early as 1827 by an English botanist, Brown, that microscopic particles in a liquid keep up incessantly a very minute trembling motion and this phenomenon remained altogether unexplained for more than half a century. At last in 1888 it was suggested by Gouy in France that this trembling motion was probably due to the fact that when a particle is sufficiently small the molecular bombardment which it receives from the molecules surrounding it is not at a given instant exactly the same on opposite sides, and in consequence the particle is pushed first in one direction and then in another by these unbalanced molecular forces. In 1908, Perrin, in Paris, with the aid of a formula deduced by Einstein of Bern, had brought forward quite convincing evidence that this explanation was correct, but Perrin's observations had all been made upon
