6 Lees, Effect of Pressure on Thermal Conductivity. 



By means of this equation k can be calculated from 

 the observations. 



In order to determine the value of h for the discs, an 

 experiment was made in which the discs under test were 

 air enclosed by a ring of silk ribbon. The heat conducted 

 through the air being small, the right-hand side of the 

 above equation is nearly zero, and the heat H is almost 

 entirely lost from the surfaces of the discs. 



The materials tested by the above method were glass, 

 ebonite, marble, slate, sandstone, and granite, of thickness 

 varying from '2 to 2 cms., according to the conductivity 

 of the material. Up to a pressure of 8oo lbs. on the 

 square inch no change of thermal conductivity was found 

 in the cases of granite and marble, a slight increase in 

 the cases of glass and ebonite, and an increase of 4 or 5 

 per cent, in the case of the soft sandstone. 



As the changes are so small that the correction due 

 to the glycerine layer is quite comparable with them, it 

 is proposed to carry out the experiments with a contact 

 film of mercury instead of glycerine, and with this end in 

 view the surfaces of the steel discs have been copper- 

 plated and amalgamated. 



