Concentration Cells in Ionized Gases, 691 



equal to — . These may be roughly determined as follows: — 



We have J? + 5 = K (6) 



and — is known for each cell ( ^ is tabulated in Table I.). 



Also, if , is the value of — when neither electrode is 

 a a. 



shielded, 



-+ -= - (0 



"o a i a 



XT 



— ; may be found from the curves which are obtained 



when no shielding Al is present fcf. gli fig. 2). From these 



curves ^='0107 and ^ =-0815. From this value and 



those in column <± Table I. we can calculate the ratio a a / a i- 

 The numbers in column 4 vary somewhat erratically and 

 were smoothed by a curve before being used. Curve C 

 (fig. 3) shows the E.M.F/s calculated in this way, but it 

 cannot be expected to be very accurate. The agreement 

 between curves A and B is, however, very satisfactory 

 considering the uncertainty of the calculation *. 



The sign of the E.M.F. of the cell — the unshielded electrode 

 being the positive pole — shows that it must be the + ion 

 which is the controlling agent in the production of the 

 potential step. The results obtained, moreover, suggest that 

 the negative ions take no part in the transfer of electricity 

 across the surface. This conclusion is in agreement with the 

 known facts of contact electricity which suggest, though no 

 absolute proof has ever been given, that a metal is ordinarily 

 at a lower potential than the air. Also the discharge of elec- 

 tricity from heated metals takes place by the emission of posi- 

 tive ions unless very high temperatures are reached. It makes 

 it, however, very difficult to understand the meaning of the , 

 solution pressure P of the initial formula. One would naturally 

 try to identify this with the pressure of the negative electrons 

 which are supposed to exist in the interior of metals, and 

 explain the potential difference between a metal and air by 

 the tendency of these electrons to escape from the metal, 

 except that this view would give E.M.F/s for concentration 

 colls of the opposite sign to those found experimentally. On 



* The curve given by Madame Curie is for /3 radiation only, therefore 

 an allowance had to be made for the y radiation. 



