63.268 10.646} } Mean = 63.325 } 63.642 } 63.020 71.664} 6 62.820 71.248} 63.514 52.668} + 63.312 52.800 7 63.776 52.496 63.300 52.860} 63.156 71.708 6 63.126 Mean = 63.407 } 63.228 42.006} } 63.294 41.920 8 63.184 42.108} 63.260 53.210} 63.478 52.922 7 - 63.074 53.034 63.306 53.438} 63.414 12.888} 63.450 12.812 19 63.446 12.748 63.556 12.824} Mean = 63.335 } Duration of experiment 1 hr. 40 min. Mean = 6.697 Initial volts = 1723.5 Mean = 6.700 Final volts = 1702.1 Pressure = 53.48 cm. [Pg 85] [Pg 86] Since this is about the limit of the experimental error (the probable error by least squares is 1 part in 1,500), we may with certainty conclude that there are no differences of more than this amount between the values of the positive and negative electrons. This is the best evidence I am aware of for the exact neutrality of the ordinary molecules of gases. Such neutrality, if it is actually exact, would seem to preclude the possibility of explaining gravitation as a result of electrostatic forces of any kind. The electromagnetic effect of moving charges might, however, still be called upon for this purpose. VI. RESISTANCE OF MEDIUM TO MOTION OF DROP THROUGH IT THE SAME WHEN DROP IS CHARGED AS WHEN UNCHARGED A second and equally important conclusion can be drawn from Table VIII. It will be seen from the column headed “” that during the whole of the time corresponding to the observations in the third group from the top the drop carried either 6 or 7 electrons, while, during the last half of the time corresponding to the observations in the second group from the top, it carried three times as many, namely, 22 electrons. Yet the mean times of fall under gravity in the two groups agree to within about one part in one thousand. The time of fall corresponding to the heavier charge happens in this case to be the smaller of the two. We may conclude, therefore, that in these experiments the resistance which the medium offers to the motion of a body through it is not sensibly increased when the body becomes electrically charged. This demonstrates experimentally the exact validity for this work of the assumption made on p. 70 that the velocity of the drop is strictly proportional to the force acting upon it, whether it is charged or uncharged.