CONDUCTIVITY. 



101 



metals there was considerable dispersion. Probably the refractive index 

 for very long waves should be compared to bring out any true physical 

 relation, and red light is only the best approximation which could be 

 made to such long waves. At present we can only say that Kundt's 

 results point in the direction of some connection between the three sets 

 of phenomena, light velocity, heat conductivity, and electric conductivity. 



Solids of Low Conducting Power and Crystals. Many experi- 

 ments have been made to determine the conductivity of non-metallic 

 solids. With amorphous solids usually some method similar to that of 

 Peclet has been used, and the range of conductivity has always been 

 found to be far below that of metals. Taking first the conductivity of 

 crystals, we might expect that this would be different along the different 

 axes, an expectation verified by experiment. The subject was first 

 studied by Senarmont, who used a very simple method, preparing a 

 plate of the crystal with a small hole through it. The plate was covered 

 with a film of beeswax, and a silver wire passing through the hole was 

 heated. The heat was conducted through the crystal, and the beeswax 



FlG. 69 (a and 6). Showing Conductivity in Crystals. 



was melted. When the conduction was the same in all directions in the 

 plane of the section, as in a plate of quartz cut perpendicular to the axis, 

 the figure of the melted wax was circular, as in Fig. 69a. When it 

 differed in different directions the figure was elliptical as in Fig. 69&. 

 The conductivities along the two axes of the ellipse may be shown to be 

 proportional to the squares of the axes. 



A method of experiment developed by Lees * is especially suitable for 

 the determination of the conductivity of crystals. A long brass bar with 

 diameter 1'93 cm. was used as in Forbes's experiment, and its conduc- 

 tivity was determined by his method as - 268{1 + -002( - 17)}. It was 

 then cut in the middle and a plate of the crystal, of the same area 

 as the cross-section of the bar, was inserted between the cut faces, these 

 being amalgamated to give good contact with the crystal. Temperature 

 observations along the bar, made by means of thermo-electric junctions, 

 gave the temperature of each face of the plate and therefore the tem- 

 perature slope through it, while the known conductivity of the bar and 

 the temperature slope in it adjacent to the plate gave the rate of passage 

 of heat into and out of it. From these data the conductivity could be 

 determined. 



Lees found the following conductivities between 25 and 35 : 



Quartz, along the axis .... '0299 



,, perpendicular to the axis . . '0158 



Iceland spar, along ,, ,, . . '0100 



,, perpendicular to ,, ,, . . *0084 



Mica, ,, cleavage planes . "0018 



* Phil. Trans., A., 1892, p. 481. 



