o 
MR. W. CASSIE ON THE EFFECT OF TEMPERATURE ON 
investigated here are amongst those for which Dale and Gladstone have observed 
the refractive indices at several temperatures. For one of these four the temperature 
effects show no similarity whatever, for another the relation is fairly close to that 
indicated by the theory, and for the other two, though not agreeing exactly with the 
theory, the relation does not differ from it very greatly, not more, perhaps, tban 
might be explained l:)y differences in the specimens used. 
Tlie experiments were done in tiie Cavendish Laboratory, Cambridge, and T am 
glad of this opportunity of thanking Professor J. J. Thomson, F.P.S., at whose 
suggestion they were undertaken, for placing the resources of the Laboratory at my 
disposal, and for valuable advice during the course of the work. 
1. Solids. 
The method adopted in the case of solids was to make a condenser with the 
substance to lie experimented upon and observe its capacity at different temperatures. 
The condenser consisted of a pile of alternate thin sheets of the dielectric and discs of 
lead foil, with flat plates of iron above and below, and a weight on the top to keep 
them together. The condenser could not be fixed together in any more permanent 
way, because the unecpial expansion with heat of the different materials would have 
altered the degree of compression of the pile, and so produced a change of capacity 
greater than the effect under investigation. 
Special precautions were required to secure that the insulation between the two 
sides of the condenser be as far as possible independent of the temperature. To 
secure this the condenser was suspended in a stirrup from a bracket about four feet 
above the air bath in which it was heated. The suspending wires, which formed the 
connection for the side of the condenser in contact with the stirrup, passed without 
touching through holes in the top of the air bath, and over a short glass tube 
varnished with shellac, which was fixed across the bracket above. The connection 
for the other side of the condenser consisted of a stiff’ wire, which also passed without 
touching through a hole in the top of the air bath, and was carried through the glass 
tube on the bracket. Thus the condenser had no contact with anything about the 
air bath, and was supported solely by the glass tidie on the bracket, which was 
always cold and perfectly insvdating. The perfect freedom of the condenser and its 
supports was easily tested before each observation by tapping the air bath and seeing 
that no motion was produced in the suspending wires. 
The condenser was heated by raising the bath quickly to a temperature considerably 
above that aimed at, and then leaving under it a small flame adjusted by experience to 
give the right temperature. The temperature was read by a thermometer passing- 
through a cork in the top of the bath. About three hours was the shortest time 
required to ensure that the condenser was uniformly heated throughout; it was usually 
left a good deal longer. 
