The idea has at various times been put forth in connection with attempts to explain chemical and cohesive forces from the standpoint of electrostatic attractions that the positive and negative charges in a so-called neutral atom may not after all be exactly equal, in other words, that there is really no such thing as an entirely [Pg 83] neutral atom or molecule. As a matter of fact, it is difficult to find decisive tests of this hypothesis. The present experiments, however, make possible the following sort of test. I loaded a given drop first with negative electrons and took ten or twelve observations of rise and fall, then with the aid of X-rays, by the method indicated in the last section, I reversed the sign of the charge on the drop and took a corresponding number of observations of rise and fall, and so continued observing first the value of the negative electron and then that of the positive. Table VIII shows a set of such observations taken in air with a view to subjecting this point to as rigorous a test as possible. Similar, though not quite so elaborate, observations have been made in hydrogen with the same result. The table shows in the first column the sign of the charge; in the second the successive values of the time of fall under gravity; in the third the successive times of rise in the field ; in the fourth the number of electrons carried by the drop for each value of ; and in the fifth the number, characteristic of this drop, which is proportional to the charge of one electron. This number is obtained precisely as in the two preceding tables by finding the greatest common divisor of the successive values of () and then multiplying this by an arbitrary constant which has nothing to do with the present experiment and hence need not concern us here (see chap. V). It will be seen that though the times of fall and of rise, even when the same number of electrons is carried by the drop, change a trifle because of a very slight evaporation and also because of the fall in the potential of the battery, yet the mean value of the positive electron, namely, 6.697, agrees with the mean value of the negative electron, namely, 6.700, to within less than 1 part in 2,000. [Pg 84] TABLE VIII Sign of Drop Sec. Sec. 63.050 } 63.186 41.728} 8 63.332 41.590} - 63.328 62.728 25.740} 62.926 25.798 11 62.900 25.510 63.214 25.806} } Mean = 62.976 } 63.538 22.694} 12} 63.244 22.830} 63.114 25.870} 63.242 25.876 11 + 63.362 25.484} 63.136 10.830} 63.226 10.682 63.764 10.756 22 63.280 10.778 63.530 10.672