8 4 



SCIENCE PROGRESS 



It becomes interesting to see whether these electrical 

 properties are properties of the particles, which as the ultra- 

 microscope has shown, break the homogeneity of the liquids. 

 A very convenient arrangement for this investigation is shown 

 in figs, ya and yb. C is the slide, on which are fastened two pieces 

 of platinum foil, AA. BB are a couple of thin pieces of worked 

 glass, and D is a coverslip. BB with AA form a shallow 

 trough. To further define the cell, two pieces of glass are 

 sometimes placed at right angles to BB. Platinum wires are 

 sealed to AA, and serve to connect with the source of potential. 



Fl?. ? 3L. 



Fig. 7 b- 



The preparations used should have a thickness of at least 

 0"i mm. (see below). 



If a little gold or silver hydrosol is placed in the cell and 

 viewed with the ultramicroscope, the usual bright points of 

 light are observed, which show the characteristic Brownian 

 movements. On connecting with the source of potential, the 

 particles are observed immediately to begin to move in parallel 

 directions ; there seems to be no lag between the application 

 of the electromotive force and the commencement of the transla- 

 tory movements. The motion is uniform, and the velocities 

 can easily be determined, by observing the time taken by the 

 particle to cross the field, the length of which is known. Thus, 

 in an experiment of Cotton and Mouton, it was found that the 

 time of passage was 20 seconds, the field having a length of 

 i'3 mm., and the potential gradient being 2 volts per mm. 

 The velocity is proportional to the electric force, 1 and in this 

 way the mobilities, or velocities under an electric force of 1 volt 



1 This usually obtains for particles moving against viscous forces, providing the 

 velocity is not too great. 



