DETERMINATIONS OF HEAT CONDUCTIVITY OF SELENIUM. 357 



are changed in a similar manner, but the relative alteration 

 produced in the former is, in general, smaller than that 

 brought about iu the latter property by the same cause. 



Although the values of the conductivity have been 

 obtained throughout in absolute units, very little weight 

 should be attached to the actual figures given for the 

 crystalline form. For it appears probable that in any pre- 

 paration of crystalline selenium, spontaneous changes take 

 place for a long time after preparation, and, in addition, 

 the slight alteration of temperature which is necessary 

 before any measurement can be made, is able, under certain 

 conditions, to produce a considerable change in the con- 

 ductivity. 



The results may be summarized briefly as follows : — 



(1) The thermal conductivity of vitreous selenium at 

 25° C. was found to lie between 0*000293 and 0*000328. 



(2) The conductivity of crystalline selenium at 25° O. 

 varied from 0*00070 to 0*00183. In general the conductivity 

 increased with the temperature of preparation, but 

 diminished with age. 



(3) In all the cases examined, the temperature coefficient 

 was positive. 



Method of Investigation. 

 The method used was that described by O. H. Lees. 1 The 

 selenium was tested in the form of a disc 6*5 cm. in 

 diameter, and of the order of 0*5 cm. in thickness. All 

 temperatures were measured by means of copper-eureka 

 thermo-couples which were carefully calibrated at the 

 commencement of the work. A year later other similar 

 thermo-couples were substituted, and new calibration 

 readings taken. The series method of measuring thermo- 

 electromotive-forces employed by Lees was not used. Each 



1 Phil. Trans., A, 1898, p. 399. 



