418 DR. C. H. LEES ON THE THERMAL CONDUCTIVITIES OP SOLIDS 



requisite thickness. The surface which had been in contact with the glass had a 

 crystalline appearance, the other not. 



The disc of sulphur and French chalk was made in the same way. It contained 

 about equal quantities of the two constituents. 



The shellac disc was cast between glass plates, and contained a few small air holes. 



The fibre was the ordinary " white fibre " of electrical instrument makers. 



As the result of these experiments, it may be stated that the tendency at about 

 40 C. of the thermal conductivities of solid substances not very good conductors of 

 heat, is to diminish with increase of temperature, at a percentage rate which appears 

 to vary with the nature of the substance. Amongst the materials tested, glass is the 

 only exception to this rule. 



The above values of the conductivities agree with those found by a different method 

 (LEES, ' Phil. Trans.,' A, 1892, p. 506) for temperatures between 25 and 35: glass, 

 00243; sulphur, -00045 ; ebonite, "00040 ; shellac, '00060, except in the case of 

 sulphur, where the large difference is probably due to the disc tested in the present 

 experiment not being wholly crystalline. 



PART II. CONDUCTIVITIES AND TEMPERATURE COEFFICIENTS OF LIQUIDS. 



Preliminary Experiments, . 



The apparatus used originally in the experiments on the conductivities of liquids, 

 consisted of two circular copper discs, U and M, cemented by rubber cement to the 

 opposite sides of a circular sheet of rubber, R, fig. 10. To the free surface of the 

 upper disc, a flat coil, C, of insulated platinoid wire was cemented, and served to 

 supply the heat required in the experiment. 



These discs were supported by means of a three-legged support, T, screwed to the 

 upper disc, above the third copper disc, L, which was provided with a raised edge. 

 The legs of the support were long enough to raise the lower surface of the disc, M, 

 about '13 centim. above the upper surface of the disc, L. The liquid, the thermal 



