. I 573 ] 



LX. Note on the Energy of a "Double-layer " Condenser of 

 Electronic Origin. By William C. MoC. Lewis, M.A., 

 D.Sc* 



ONE o£ the most characteristic properties of colloidal 

 solutions and emulsions is that the colloid particles and 

 emulsion particles show a movement in one direction or the 

 other when the solution is placed iu an electric field. These 

 particles are therefore electrically charged. To the class 

 carrying negative charges in water belong colloidal metals 

 prepared electrically (gold, platinum, silver, mercury), some 

 sulphides (such as arsenic trisulphide and antimony trisulphide), 

 suspensions (such as selenium and sulphur) and emulsions 

 such as rubber-latex and hydrocarbon oil particles. To the 

 class carrying positive charges in water belong colloidal 

 hydroxides in general. The s Is of common metals such as 

 lead, bismuth, and iron, are also charged positively but it is 

 practically certain that in these cases we are really dealing 

 with the hydroxides. 



From a determination of the velocity of migration of these 

 substances under a known potential gradient one may calcu- 

 late the p.d. which exists between each particle and the liquid 

 medium surrounding it from considerations first, put forward 

 by Lamb, and founded on the Helinholtz double-layer theory. 

 Lamb's expression as modified and employed by Burton f (to 

 whom we owe a great deal of our knowledge of the subject) 

 is 



v __ 4-7T rjv 



where 



V= the r.D. between the particle and the medium. 

 K= the dielectric constant of the medium. 



7)= the viscosity of the medium. 



v= the migration velocity of the particles in cm. /sec. 

 under a potential gradient of X units per cm. 



The following table summarizes the results obtained with 

 some typical colloids, emulsions, and suspensions. The par- 

 ticles in such cases are usually designated by the term "the 

 disperse phase." As regards the size of these particles we 

 find great variation even in the same preparation. Emulsion 

 particles (oil, for example, in suspension in water) have an 

 average diameter of 4xl0" 5 cm. Burton obtained values 



* Communicated bv the Author. 



t Burton, Phil. Mag. xi. p. 4l'-j (1006). 



