Heat of Granite and Basalt at High Temperatures. 



81 



be fitted accurately together without any cement. These 

 grooves were only just large enough to take the wires, 

 small enlargements being made in the centre in which 

 the junctions just fitted. As one groove was cut in a basalt 

 disk and the other in a granite one and their depth was 

 only about O'l mm., no allowance was made for them. No 

 •cement was used in putting the whole together, the asbestos 

 on the heater and on the copper disks being wetted, and all 

 the disks being pressed into close contact with each other by 

 means of a 5 kilogram weight. The lower copper disk was 

 supported inside the sand vessel by a glass ink-jar and a 

 similar one inverted over the upper copper disk carried the 

 weight, which thus was not in close thermal contact with 

 the disks. 



The results are shown in Table VI. in which K x and K 2 

 .are the conductivities of granite and basalt respectively. 



Table VI. 









K x 



9. 

 47° C 



K r 



K,. 



K 2 



2-64 xlO- 3 



207X10- 3 



1-275 



108 



2-67 



213 



1-255 



163 



2-63 



2-17 



1-21 



213 



2-76 



2 38 



1-16 



355 



2-68 



2-52 



1065 



517 



2-80 



2-75 



102 



It is evident that these values are much too low. This 

 Is almost certainly due to the thermal resistance of the 

 various minute air-films which must occur in spite of the 

 attempts to reduce them. These minute crevices exist not 

 only between the rock disks but also between the inner and 

 outer junctions and the faces whose temperatures they are 

 supposed to measure. Their effect will probably vary with 

 the temperature and, as it is qnite probable that radiation plays 

 an important part in the transference of heat across them, 

 their thermal resistance probably decreases as the tempera- 

 ture is raised. Hence the method is not suited for measuring 

 either the absolute value of the conductivity of a rock or its 

 variation with temperature, but there seems to be no reason 

 to doubt its evidence that basalt is a considerably worst 4 

 conductor than granite at low temperatures, and that the 



Phil. Maq. S. 6. Vol. 27. No. 157. Jan. 1914. G 



