422 On the Thermal Conductivities of Crystals, fyc. [Feb. 4, 



experiments were originally intended to furnish data for a similar 

 comparison for crystals, but that their object has been extended. 



After some preliminary experiments, he adopted the " divided bar" 

 method, which consists in placing a disc of the material the con- 

 ductivity of which is required, between the ends of two bars of 

 metal placed coaxially, heating one end of the combination, and 

 observing, by means of ther mo- junctions applied to the bars, the 

 distribution of temperature along them, first, with the disc in position, 

 second, with the bars in contact without the disc. When the con- 

 ductivity of the bar is known, these observations suffice to deter- 

 mine that of ; the disc. 



The bars used were 1'9 cm. diameter, and about 34 cm. long. 

 The ends which came in contact with the discs were amalgamated, 

 as this was found to be the best method of securing good contacts. 

 These bars were suspended horizontally in a frame, by means of 

 strings passing over adjusting screws, which enabled the bars to be 

 set accurately in the required position. The temperatures were found 

 by means of a copper-platinum-silver junction applied to points along 

 the bars, at which small conical holes about 0'5 mm. diameter, 

 containing mercury, were placed. This junction was in circuit with 

 a galvanometer, and the circuit was so arranged that its resistance 

 could be found by a modification of Thomson's bridge method. 



The conductivity of the brass bar was determined before cutting, 

 by the method due to Forbes of determining the loss of heat from 

 the surface by allowing the bar to cool and observing the change of 

 temperature with time, and then observing the steady distribution of 

 temperature along the bar when heated at one end. 



The author shows that change of both the " internal " and " external " 

 conductivities with temperature must be taken into account in the 

 equation for the distribution of temperature. He takes each to be a 

 linear function of the temperature, and finds finally the conductivity 

 of the bar to be 0'27 c.g.s. unit, and to increase slightly with the 

 temperature. 



The discs used were of the same diameter as the bar, and were of 

 various thicknesses, in order to make the distribution of temperature 

 throughout the bars nearly the same in each case. 



The following are the results obtained, the conductivities of a few 

 other bodies being given, in order to show the positions of the bodies 

 experimented on amongst conductors generally. No relation of the 

 kind found by Kundt for metals seems to hold for the crystals 

 experimented on : 



