20G REPORTS ON THE STATE OF SCIENCE. 



capillary tube for the diaphragm, confirmed these laws, and sliowed 

 further that the equilibrium -pressure increased with diminished bore 

 and relatively increased surface of the capillary ; and that not only the 

 pressure, but the direction of the flow were dependent on the nature of 

 the material of the capillary and the liquid employed ; oil of turpentine 

 flowing to the anode in a glass capillary, and in the reverse direction in 

 one lined with sulphur. It is obvious, therefore, that the action is 

 between the surface of the liquid carrying the electric current and that 

 of the capillary, and that if the liquid were fixed and the capillary 

 surface free to move, it would be carried in the opposite direction to 

 the flow of the liquid. This will be the case with suspended particles ; 

 and it has long been observed that these are carried in one or other 

 direction according to their material and that of the liquid, most sub- 

 stances moving toward the anode in water and the cathode in turpentine, 

 with a velocity proportional to the current-strength, but independent 

 of their distance from each other, and of the electromotive force. 

 Quincke assumed that the particles possessed electrical charges different 

 from the liquid, and usually negative as compared with aqueous solutions ; 

 so that in a current the — particles would tend to move to a region of 

 higher -f potential, that is, towards the anode. The relation of these 

 facts to the contact- theory and to the phenomena of frictional electricity 

 is obvious ; a stream of liquid forced through a capillary of different 

 potential would necessarily give rise to an electric current if furnished 

 with appropriate conductors. 



Helmholtz supported and somewhat amplified the hypothesis of 

 Quincke.^ According to him, the limiting surfaces of two bodies of 

 opposite electric character form a ' double layer ' (Doppelschicht) ; the 

 -\- charge of the one layer exactly equalling the — charge of the other, 

 and thus exerting no outside effect. Under the influence of a fall of 

 potential the liquid layer tends to move over the other, and by friction 

 communicates its motion • to the adjacent liquid. Billitzer '^ supposes 

 that the liquid coating of the double layer communicates its charge by 

 diffusion to the remainder of the liquid, and, as this charge is constantly 

 renewed by contact-action, both liquid and particles assume permanent 

 charges, so that the' latter behave, in a certain sense, like free ions. 

 It is possible, or indeed likely, that the pai'ticles owe their original 

 charges to the actual attachment of free ions by their residual affinities. 

 It is not necessary to suppose, or even probable, that when two or more 

 atoms unite to form a molecule their affinities are quantitatively saturated, 

 but merely that, under the conditions, the residual atiJnity is insufficient 

 to attach another atom. Under these circumstances it is likely that a 

 mass of molecules may be able to hold additional ions in a way probably 

 identical with what is known as 'adsorption' ; and these ions will affect 

 the charge of the mass. The bearing of this on the phenomena of 

 tiocculation will be referred to later. 



The phenomenon of electric cataphoresis was first studied with fine 

 mechanical suspensions, and has been employed technically for clearing 

 turbid liquids, and more recently for drying pasty mixtures, such as 

 alizarine paste or ground peat ; currents of high potential being used. 

 It is, however, very marked in colloid sols, and closely simulates actual 



' Ann. Phy$. (3), 1879, t, 337. 

 » Wien. JBer., 1903. 113. 9S-139, 



