CONDUCTIVITY. 



103 



Knowing h from the equation for the heat supply H, we can hence find 

 K. We have omitted all account of corrections. For these the original 

 paper may be consulted. 



The method gave the conductivity at different temperatures, and 

 assuming that it might be put in the form 



the values which Lees found for the conductivity at 35, and for the 

 co-efficient a are 



Window glass 

 Sulphur . 

 Ebonite 

 Shellac . 



00245 

 00067 

 00042 

 00058 



+ 0025 



- -0036 

 -0019 



- -0055 



We add some conductivities which he found by the divided bar 

 method for a mean temperature of 30 : 



Marble . 



Slate . 



Paraffin 



Paper . 



Silk 



Mahogany 



Cork 



00709 



00475 



00061 



000315 



00022 



000465 



000129 



The following conductivities are taken from Lord Kelvin's article on 

 Heat in the Encyclopedia Britannica (9th ed.). They probably need 

 revision : 



Substance. K 



Authority. 

 'Forbes and Lord Kelvin ob- 

 tained by observations on 

 underground thermome- 

 ters, noting the delay of 

 the heat of summer and 

 the cold of winter. 



Ayrton and Perry. 

 Peclet. 



Sandstone of Craigleith Quarry -01068 

 Underground strata . . '005 

 Trap rock of Calton Hill . -00415 



Porphyritic trachyte . . '0059 



Oak across fibres . . . -00059 



Fir along fibres . . . -00047 



Fir across fibres . . . -00026 



Cork -000029 



Writing paper . . . -000119 



Carded wool .... '000122 



Finely carded cotton wool . -000111 



Eider-down .... '000108 



Conductivity Of Liquids. Usually conduction in a liquid is 

 greatly assisted by convection. If, however, convection be prevented 

 by heating the liquid from the top, it is found that except in the case of 

 mercury the conducting power is low. The first quantitative experiments 

 appear to have been made by Despretz, who heated a column of water 

 from above, and observed the temperature at various depths when the 

 steady state was reached. He found that the slope of temperature was 



