THE KINETIC THEORY OF MATTER 431 



tiiDles of the smallest charge caught from the air. Some of these drops 

 have started with no charge at all, and one, two, three, four, five and six 

 elementary charges or electrons have been picked up. Others have started 

 with seven or eight units, others with twenty, others with fifty, others 

 with a hundred, others with a hundred and fifty elementary units and 

 have picked up in each case half a dozen elementary charges on either 

 side of the starting point, so that in all oil drops containing every pos- 

 sible number of electrons between one and 150 have been observed and the 

 number of electrons Avhich each drop carried has been accurately counted. 

 It is not found possible to count with certainty the number of electrons 

 in a charge containing more than 200 of them, for the simple reason that 

 the method of measurement used fails to detect the difference between 

 200 and 201. But it is quite inconceivable that large charges such as 

 are dealt with in the commercial applications of electricity can be built 

 Tip in an essentially different way from that in which the small charges 

 whose electrons we are able to count are found to be. Furthermore, 

 since it has been definitely proved that an electrical current is nothing 

 but the motion of an electrical charge over or through a conductor, it is 

 evident that the experiments under consideration furnish not only the 

 most direct and convincing of evidence that all electrical charges are 

 built up out of these very units or electrons which we have been dealing 

 with as individuals in these experiments, but that all electrical currents 

 consist merely in the transport of these electrons through the conducting 

 bodies. 



The next important question which the above method of experi- 

 menting seemed calculated to throw additional light upon is, " What 

 does the ionization of a gas molecule consist in?" Since it is now 

 practically certain that a molecule of air, that is a molecule of nitrogen 

 or oxygen, contains at least a hundred electrons, and possibly very many 

 more, the act of ionization might consist in the knocking out from a 

 single one of these molecules of a large number of electrons, or it might 

 consist in the complete shattering of an atom by some sort of explosive 

 process ; or, on the other hand, it might consist merely in the detaching 

 of a single electron from a neutral molecule, thus leaving the molecule 

 essentially the same sort of thing that it was before the ionization took 

 place, save that it has acquired an amount of electricity of the opposite 

 sign equal to that of the charge detached. Some little light can be 

 thrown on this question by studying the observations already presented. 

 In these observations, however, all the changes of charge took place 

 when the drop was falling under gravity, that is, when the electrical 

 field was ofP, and this for the reason that the chance which a drop has of 

 capturing an ion when the field is ofE is enormously greater than its 

 chance of catching one when the field is on, since, in the latter case, the 

 electrically charged fragments of an atom, formed by the ionization of a 



