tiples 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 possible number of electrons between one and 150 have been observed and the number of electrons which 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 experimenting 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 off, and this for the reason that the chance which a drop has of capturing an ion when the field is off 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