THE KINETIC THEORY OF MATTER 435 



it was off. Here, then, is an ahsoluteJy direct proof that the ion must 

 he endowed ivith a hinetic 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 unneutralizecl electiical 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. Kow this is exactly 

 what was observed to be the case, and, indeed, in one instance, a rela- 

 tively heavily charged drop was watched for four hours, during which 

 time it succeeded in picking up hut 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 ^Jositive ions of the gas to overcome their kinetic 

 energy of agitation when they sliot 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 mathe- 

 matical or physical theory. Fortunately the trail had already been 

 blazed and nothing had to be done but to clear out some of the remain- 

 ing 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 explana- 

 tion was correct, but Perrin's observations had all been made upon 



