"Double- Layer" Condenser of Electronic Orlyin. 575 



Now as to the mechanism whereby the p.d. is set up, we 

 find two views held — first, that it is an electrolytic ion effect; 

 secondly, that it is electronic. For the first view it is evident 

 that we must assume different electrolytic ions coining into 

 play as we alter the medium. The chief objection is that 

 colloidal solutions can be prepared from substances with 

 which it is extremely unlikely to suppose electrolytic ions 

 associated — such, for example, as colloidal platinum in chlo- 

 roform. The electron view, on the other hand, is applicable 

 in all cases, and the absence of specific effect noted above is 

 very strong evidence in its favour. It must be confessed 

 that it is by no means easy to form a mental picture of the 

 mechanism whereby a double layer can be maintained by 

 the emission of electrons. Perhaps the equilibrium is a 

 dynamic one, the potential finally reached being determined 

 by equality between the number of electrons emitted by the 

 particle and those returning to it. 



Whatever the mechanism may be it may be of interest to 

 point out a relationship which appears to hold between the 

 electrostatic energy of the double layer (regarded as equiva- 

 lent to a small condenser, the plates of which are molecular 

 distance apart) and the kinetic energy of the electrons. 



According to the prevailing theory of metallic conduction 

 metals contain a certain number of free electrons in tempe- 

 rature equilibrium with their surroundings. At 0° C. the 

 average velocity of an electron is 10 7 cm. per second (' Cor- 

 puscular Theory of Matter,' p. 52). Now suppose such an 

 electron escapes from a colloidal particle and is brought to 

 rest after traversing the distance corresponding to the thick- 

 ness of the double layer. The decrease in kinetic energy is 

 ]mr 2 . The increase in electrostatic energy is \ire, where e 

 is the charge on an electron, and if we equate these expressions 

 and solve for ir, 

 mv 2 

 e ' 

 m= 2ooo °f * ne hydrogen atom = 5 x 10~ 28 gram, 



e— 10~ 20 electromagnetic unit = 3 x 10~ 10 electrostatic unit, 

 and therefore 



7r= l'6xl0 -5 electrostatic unit 

 = 0048 volt, 

 ivJiich is in fair agreement with the observed P.D. of colloidal 

 metals. 



Chemical Laboratory, 



University College, 

 Cower Street, London, W.O. 



