[burton] 



COLLODIAL PARTICLES 



105 



in the other will sink. In practice, the connections may be made 

 through a reversing key and the voltage, usually fixed at about 110 

 volts, may be left on one way for ten minutes and then reversed for 

 twenty minutes. The velocity is reckoned from the displacement of 

 the surfaces during this final twenty minutes; one-half the sum of the 

 displacements in the, two tubes taken as the distance travelled by a 

 particle in the given time. A typical set of observations is given in 

 Table I. 



Table I 



Electrodes at 15 mms. in each limb. 



"It will be seen from the table that there has been an apparent 

 settling of the colloid in the tube while the current was running. This 

 is quite usual, but, as the reckoning is made, it could not affect the rate, 

 since this slight lowering of the surface is uniform in both limbs, so 

 that while it is added to the velocity in one limb it is subtracted from 

 the velocity in the other." 



The settling referred to above was at the time looked upon as 

 merely a disturbing circumstance. The true importance of it has 

 just recently appealed to the writer. It will be seen from the table 

 that during the first 10 minutes, the colloid moved up in the right side 

 and down in the left side of the tube; during the second ten minutes 

 the motion was reversed. From the figures quoted, which were typical 

 of a large number of experiments, there was a settling in each tube of 

 one mm. It is quite apparent that this settling is due to the action 

 of gravity. As in the case of Millikan's^ work with single droplets 

 of liquids in air, we are dealing here with a motion which is caused 

 during one period of 10 minutes by the force Xe + mg and during the 

 second 10 minutes by Xe — mg, where X is the electric field maintained 



1 Millikan. The Electron. Phil. Mag., June, 1917. 



Sec. III. Sig. 8 



