68 Mr Oxley, Magnetic Susceptibility with Temperature. 
(a) Paramagnetic susceptibility. 
Let n, be the number of particles of type p per unit mass of 
the solution. Let S be the absolute temperature and C, the 
Curie constant per particle of type p. Since the susceptibility of — 
salt solutions, even of the ferromagnetic elements, is independent — 
of the intensity of the magnetic field and there is no hysteresis 
effect, it is not necessary to take into account the magnetic — 
influences of the particles on one another. 
With these assumptions we may write the specific para- 
magnetic susceptibility 
There is considerable evidence that the complex groups of 
molecules which are known to exist in solutions vary in com- 
position as the temperature changes. Therefore n, is a function 
of the temperature. 
Write ip = Top FS) (2). 
Mp is the number of particles of type p which are found in unit 
mass of the solution at a temperature 3, given by 
F,(&) = 1. 
Be Wlpa Ges 
Therefore Xp = er, ear F'(S)> aces ecsameeneeeeee (3). 
We shall refer to a series of researches by J. J. van Laar for | 
the purpose of investigating the nature of the function F, (9). 
In his work on the theory of the liquid* and solid states, he takes 
into account the process of association, and shows that in the case 
of a substance composed of simple and double molecules, the 
degree of dissociation of the double molecules which exist at any 
temperature 3 is given by the equation 
2 — U2 w 
oes =O SRB oD 42), o (Pte) ae (v—b) ...... (4), 
a 
where (P 4 ) @=b)=0 48) Rees (4). 
Here £, is the degree of dissociation of the double molecules, 
yf is the change of specific heat when one gramme molecule of 
double molecules passes into two gramme molecules of simple 
molecules, keeping the volume large and constant. Ab, is the — 
accompanying change in the volume of the molecules, q, is the 
quantity of heat absorbed in this change at the temperature 
S¥=0; P, v, a, b, R and & have the usual interpretation as in ~ 
van der Waals’ equation. 
* Arch. Teyler (2), t. 11, troisiéme partie, pp. 235—331, 1909. 
