1 8 Mr. C. H. Lees on 



velocity of transmission of light and thermal conductivity, 

 make it important to determine the conductivities of 

 crystals in different directions with greater accuracy and in 

 absolute measure, in order to compare different crystals with 

 each other. At the suggestion of Prof Kundt, I commenced 

 some time ago at Strasburg a series of experiments with this 

 end in view. Although these experiments are not yet com- 

 pleted, I propose in this paper to give a short account 

 of the methods tried and the approximate results obtained. 



The desirableness of dealing with small pieces of crystals, 

 and the impossibility of making temperature observations in 

 the crystals themselves, point at once to placing a thin plate 

 of a crystal between the ends of two bars of metal, one 

 heated and the other cooled at the ends away from the plate, 

 and observing the temperature along each bar. These 

 observations, if the conductivity of the bars were known, 

 would furnish data for a determination of the conductivity 

 of the crystal. 



Difficulties arise, however, from the imperfect nature of 

 the contacts between metal and crystal. Lodge* proposed 

 to improve them by inserting pads of tin-foil, but this I 

 found not to answer. Eventually a metal which would 

 amalgamate was used for the bars, and the amalgamated 

 ends made contact extremely well. 



As the bars and crystal lose heat by convection, conduc- 

 tion, and radiation to the surrounding air, a method (that 



o 



of Angstrom) was first tried which gives by one experiment 

 both the internal and external conductivities. One end of 

 the arrangement of bars and crystal was alternately heated 

 for six minutes by steam and cooled for six minutes by 

 water till the temperature throughout was a periodic function 

 of the time. 



* Prof. O. Lodge proposed in 1878 (/%z7. Mag. [5] V. p. no) to deter- 

 mine the conductivities of crystals by the above method, but I am not aware 

 that he has published any results. 



