CONSTITUTION AND TEMPERATURE ON MAGNETIC SUSCEPTIBILITY. 
201 
H be zero or not. It is really due to the spontaneous local intensity of magneti¬ 
zation per unit volume and corresponds to the similar energy term, (Weiss), 
due to the spontaneous magnetization in iron. Using the values which have already 
been assigned to a\ and viz., 
a!^ = 2'5 X lO^t ] 
m4oo }■. 
we see at once that this term is large in comparison with and tlierefore the 
change of volume to which the potential energy term a',,, P gives rise will he large 
in comparison with that which we can produce artificially in a liquid by applying the 
largest field available in the laboratory. 
When a diamagnetic substance crystallizes, the alteration of Internal pressure will 
he \a '^. V and therefore the accompanying change of volume in cubic centimetres per 
cubic centimetre will be 
W = i.c.<.P, 
and substituting from (4) we find 
W = ^ . 0-8 X 10-1° X 16 X 10' X 2-5 X IQi = 0-16 c.c./c.c. 
to the appropriate order. 
Some values of SV in cubic centimetres per cubic centimetre are :— 
Substance. 
SV. 
Benzene. 
. . . 010 
Naphthalene . 
. . . 014 
Benzoj)henone. 
. . . 019 
Di-phenylamine . 
. . . 010 
Formic acid 
. . . 010 
Sodium. 
. . . 0-03 
Potassium .... 
. . . 0-03 
Mercury .... 
. . . 0-036 
Lead. 
. . . 0-03 
Tin. 
. . . 0-03 
Bismuth .... 
. . . 0-03 
Iron (at A3 point) 
. . . 0-003 
The values calculated agree as well as could be expected with the experimental deter¬ 
minations, since we know the orders of magnitude only of a'^ and I, for these are 
* See p. 257 supra. 
t A. physical explanation of this large value of a'c and of the corresponding constant N in ferro¬ 
magnetism is given on p. 267 infra. 
1 G. Tammann, ‘ Kristallisieren u. Schmelzen,’ Leipzig, 1903, pp. 204 et seq. 
§ Desch, ‘ Metallography,’ p. 242. 
