(154) 
Electrode of grey | Solution of a tin | Electrode of white 
tin. | salt. | tin. 
In the figure, a and b are glass tubes, 7 cm. long and 14 em. 
wide united by the wide middle-piece c. 
The grey modification of the tin is placed in a, the white in b. 
In contact with the powder in each tube is a platinum wire, 7; and 
rg, Which is fused into a glass tube and bent into a ring at its 
lower end. An electrode made in this way has many advantages 
in practice !). 
An aqueous solution of a tin salt is poured into a, b and c¢, and 
a and & are closed with corks, #; and Z, which allow the wires 7 
and r, to pass. The whole element thus formed may be suspended 
in a thermostat by means of the glass rod g which is fused to it. 
2. The theory of this element is easily given and offers, as will 
appear, many points of interest. 
If an electrode of grey tin is placed in a dilute solution of a tin 
salt, in which the tin ions have an osmotic pressure p,, the poten- 
tial difference between the electrode and the solution at the tem- 
perature 7 is 
RE B, 
= Sn ae 
n Eg P1 
where » is the valency of the tin, «, the number of coulombs attached 
to 1 gramion, P, the electrolytic solution tension of the grey tin 
at the temperature 7 and A the gas constant. 
If an electrode of white tin is now placed in the same solution 
we obtain 
RL Pw 
> 109. 
"Eq at 
The E.M.F. of the transition element so obtained is then repre- 
sented by the equation 
: RF . P 
KE == Ey — Lf, == log. : Volt == 
n Eq w mn 
0.0061983 Py 
ER A Volis. … 3} 
w 
0,0001983 7 . 
e 
Sine - Is a constant at a given temperature we see 
n 
1) See RICHARDS and Lewis, Zeitschr. für phys. Chemie, Bd. 28, S. 1 (1899). 
rf 
