116 
MR. A. E. OXLEY ON THE INFLUENCE OF MOLECULAR 
surrounded by glass tubes, TXT, to prevent disturbances due to air currents. A 
screen J, formed of two pads of asbestos board, with an intervening layer of cotton¬ 
wool kept wet by water from the pipe p, served to prevent disturbances due to air 
currents from the walls of the electric furnace F. The temperature attained was 
measured by the copper German-silver couple, T : fitted into the top of the furnace, as 
shown. A support of wood W was inserted between the phial and the main brass 
suspension to prevent conduction of heat and the accompanying thermal disturbances. 
In both series of experiments the whole apparatus had to be surrounded with pads of 
cotton-wool to keep a steady zero. 
(4) Discussion of Possible Errors. 
In the measurement of magnetic properties over wide intervals of temperature it is 
necessary to consider the effect of a number of possible sources of error and these will 
now be considered. 
(a) The Effect of Alteration of Volume during Fusion or Crystallization on the 
Volume of A ir Displaced by the Substance. 
We shall denote the specific susceptibility of the liquid by xo that of the crystals 
by Xc- From equation (y), by division, 
— dj X.-M— x„(M„ + 3M„) = l + M9x — Xa • 
cl 
d l -d 1 
Xi 
M —XaM 
Xi 
M —XaM 
1 + ' ri \,r + where ox = Xc _ Xc 
xu 
M— X «M a 7-0x10- 
oM c is the additional mass of air displaced owing to the change of volume of the 
substance in passing from one state to another, 3V a the corresponding change of 
volume, and fa is the susceptibility of air per unit volume. (0'25 x 10"'). 
For most substances, cV n = 0T,t if V a = 1, i.e., for the volume of substance 
experimented on in this research. Hence the variation of deflection due to this 
cause is 0'25 x 10 -7 x OT/7’0 x 10~ 7 , or 4 parts in 1000, and is beyond the limit of 
experimental accuracy. In the above it is assumed that there is no appreciable 
change of temperature of the system while fusion takes place, so that no effect due to 
variation of x a with temperature enters. 
* - 7'0 x 10 -7 is taken as a mean value of xi■ M - x«M 0 . 
t Fide G. Tammann, ‘ Kristallisieren u. Schmelzen,’ Leipzig, 1903, pp. 204 et seq. The values of 0V in 
cubic centimetres per cubic centimetre for some of the substances investigated in this research are:— 
Benzene, 0 ■ 10 ; Naphthalene, 0 • 14 ; 
Benzophenone, 0 - 09; Mercury, O'036, 
Nitrobenzene, O'08, 
