LUMINOSITY OF FLAMES CONTAINING SALT VAPOURS. 
79 
Replacing F — m by F and eliminating n and m from (l), (2), and (3), we obtain 
8 + dr- = Ac. 
1 + a Ac /3Ac 
If we take c — 1 when S = 0 this becomes 
or 
S = Ac _ A 
1 + a Ac c 
S = A — (1+aAc).(4) 
When c is large this reduces to 
S = aAV. 
Since all salts give ions having the same velocities in the flame, it follows that the 
constant A should have the same value for all salts. The equation (4), therefore, 
only contains one constant (a) which varies with the nature of the salt. 
The fraction of the salt molecules which are ionised is given by 
n 
1 
S 1+aAc 
Let k denote the concentration in grammes per litre of the solution sprayed into 
the flame, and let GS = 10 4 &, where G is a constant proportional to the molecular 
weight of the salt. 
We have then 
A 2 —1 
10 4 £ = GA 
(l + a. Ac). 
When c is large suppose 10 4 & = ac 2 , so that a — GaA 2 , and let b = GA, so that 
10 4 & = 
c 2 — 1 
(6 + ac) 
and 
n 
S b + ac 
(5) 
( 6 ) 
The equation (5) agrees with the values of c found for csesium chloride within the 
limits of experimental error if b = 10 and a = 1. The following table gives the 
VOL. CCXVI.-A. M 
