CONSTITUTION AND TEMPERATURE ON MAGNETIC SUSCEPTIBILITY. 
113 
The displacement of the substance, due to the action of the force X, was usually 
about Trj^y mm. This will not appreciably alter the sensitiveness of the bifilar 
suspension, and, therefore, the use of torsion to bring the 
body back to its initial position is not necessary. 
The substance was placed in a small glass phial (fig. 3). 
Form (a) was used for liquids, form (b) for both solids and 
liquids; (a) was provided with inlet and outlet tubes of fine 
bore, which admitted only a minimum contact of the sub¬ 
stance under investigation with the air, and thereby 
reduced as much as possible any effect due to evaporation. 
The stopper consisted of a short piece of indiarubber tubing 
closed at one end by a glass plug. By this device the bulb 
could be thoroughly cleaned and easily filled. To fill the 
phial, the stopper was removed and the liquid drawn into 
the bulb. By tilting the bulb slightly, the liquid ran out of 
the shorter limb and the stopper was placed in position. The 
liquid in the longer limb was removed by means of a fine 
capillary tube. In this way it was possible to obtain a sphere 
of the liquid under investigation. This type of phial has been 
used with some substances which are crystalline at ordinary 
temperatures by filling the phial in a high temperature enclosure. The type (b) was 
devised primarily for use with crystals which could not be introduced into (a). It 
has been used for liquids as a check against any effect which might arise from the 
motion of the liquid in the lower limb of type (a). 
FIG. 3 
Crystals introduced at arrow 
Let d x be the deflection due to phial and contained air. 
,, ,, ,, ,, ,, water. 
,, d ,, ,, ,, „ substance. 
,, specific susceptibility of air. 
>» Xa 
)) 
» X 
water. 
substance. 
Now X = pD, where D is the deflection on the scale, and p is some constant, 
provided the deflection of the torsion arm is small. Hence from (4) 
P (d-d) = (H,' 11 + H„ + H, aH 
dx 
dx 
dx 
(a) 
P(d-d 1 ) = ( x M— Xa M a )(H. 
cHy , JJ 
y ex 2 dx / 
dx 
(|S) 
if the phial be placed in the same position relative to the pole pieces. Here M is the 
mass of the substance taken, that of the air which fills the same volume, rn a is the 
Q 
VOL. CCXIV.-A. 
