374 SCIENCE PROGRESS. 



silver from a solution of silver nitrate. When it is re- 

 membered that the mass of the atom of hydrogen is to that 

 of silver as i : 108, it is evident that the same quantity of 

 electricity always passes when one univalent atom is set free. 

 In the case of divalent atoms like oxygen, we find that one 



r , . . ... Atomic Weight 

 unit oi electricity liberates 0*0001035 x 2? — 



grams, hence the quantity of electricity which is needed 

 to liberate one divalent atom is twice as great as that re- 

 quired to set free one univalent atom. This definite relation 

 between the quantity of electricity passing through the 

 solution, and the number of atoms liberated, is easily repre- 

 sented by supposing that each univalent atom, while it is 

 travelling through the solution as an ion, carries a definite 

 charge of electricity — of positive electricity if it is a kation, 

 of negative electricity if an anion — which it gives up to the 

 electrode when it is reached, while each divalent ion carries 

 twice as much, and so on. The passage of a current 

 through a solution can therefore be looked upon as a kind 

 of convection. A constant stream of kations carries positive 

 electricity in one direction, and a corresponding stream of 

 anions carries negative in the other. 



Suppose we have N gram-equivalents of salt, and conse- 

 quently N gram-equivalents of both anion and kation, in each 

 cubic centimetre of the solution. If the velocity of the anion 

 be v and that of the kation u, the numbers of them passing 

 in one second across each square centimetre of area in a 

 plane at right angles to the direction of their motion are 

 Ne; and Nw respectively, so that the quantity of salt de- 

 composed per second is N (u + y) or NV gram-equivalents 

 where V is the relative velocity of the ions past each other. 

 Now by Faraday's law we know the quantity of electricity 

 which passes when one gram-equivalent of salt is decom- 

 posed : it is units. The quantity of electricity 



r 0*0001035 



which passes in one second through unit area of cross 

 section of the solution at right angles to the flow is there- 

 fore ■ units. But this gives the strength of the 



0*0001035 



