426 DR. 0. H, LEES OST THE THERMAL CONDUCTIVITIES OF SOLIDS 



question then arises : Is there any sudden change in the conductivity as the substance 

 passes from one physical state to another, or is this change continuous ? 



In order to test what change occurs in the thermal conductivity of a substance as 

 it passes from the solid to the liquid state, the apparatus used for liquids was 

 modified slightly, so as to allow space between the lower and middle discs for change 

 of volume of the substance tested on melting. 



The ebonite ring was replaced by one of "white fibre," pierced by a hole 

 4M 6 centims. in diameter, i.e., greater in diameter than the middle copper disc (Kg. 13), 



Fig. 13. 



with three strips, S, of fibre projecting inwards, in order to provide support for the 

 middle disc, and keep the layer of substance tested of constant thickness. By means 

 of a wire frame attached to the lower disc, and a screw, the upper discs were pressed 

 down on to the strips of fibre, and the lifting of the upper discs, owing to expansion 

 of the substance tested on melting, prevented. 



Some of the substance to be heated was placed within the ring, and the lower disc 

 heated till the substance melted. The upper discs were then placed in position and 

 the apparatus allowed to cool. The experiment was then conducted as in a test of 

 a liquid. 



The area of the projecting lugs of the fibre disc, through which heat could flow, 

 was estimated as approximately '4 centim., hence the heat transmitted per second per 

 unit slope of temperature through the fibre, and not through the substance under test, 

 would be = "0003 gram degrees approximately.* The nett area of flow through the 

 substance has been taken 13'3 sq. centims., but as this number is, to some extent, 

 uncertain, owing to the difficulty of obtaining discs which, in the solid state, com- 

 pletely filled the space within the fibre ring, the results given in the following table 

 for different substances are not comparable with each other to within 3 or 4 per cent., 

 although those for the same substance, at different temperatures, are. 



* The conductivity of the fibre = '0008 (pp. 417, 418). 



