1 8 NEWBERY, The Theory of Over-voltage. 



The colloidal residue from aloes after the extraction 

 of aloin is finding considerable application in electro- 

 deposition of certain metals, as it tends in a high degree 

 to induce the metal to form smooth coherent layers, 

 instead of the rough uneven surfaces normally obtained. 

 No parallelism has been found between this effect and the 

 effect on the overvoltage. 



When a metal such as lead has been used as electrode 

 in an electrolyte containing a colloid which raises its over- 

 voltage, it retains its raised overvoltage when washed with 

 water and used in a pure electrolyte. Even lightly scrap- 

 ing the surface is not sufficient to destroy this high over- 

 voltage, though heavy scraping does so at once. We are 

 bound to conclude therefore that the colloid penetrates to 

 an appreciable depth into the electrode surface, and it 

 does not exert its full effect on the overvoltage until this 

 penetration has taken place. 



The following idea of the action of colloids during 

 metal deposition may appear somewhat fantastic, but it 

 certainly has the virtue of possibility. 



Colloid molecules differ from others chiefly in their 

 very high molecular weights, most of which are probably 

 several thousand. As shown above, the ions by virtue 

 of the attractive power of their electric charges, carry 

 along with them some of these heavy molecules up to and 

 into the metal surface. When these colloid carriers get 

 near the electrode they will move much more slowly than 

 free ions and will be thus under the influence of the 

 electrical attractive force for a longer time. This will 

 produce a high momentum in the colloid and carrier 

 which will strike the electrode with considerable force. 

 A continual hammering action by big molecules, which 

 in many cases lose their carrying ion and rebound from 

 the surface as soon as the ion gives up its charge, can well 



