82 Conductivity and Specific Heat of Granite and Basalt. 



conductivities become nearly equal at about 500° G. This 

 agrees well with the results obtained in the second basalt 

 determination of the cylinder method. 



Conclusion. 



There seems to be little doubt that the conductivity of 

 granite falls from nearly 6 x 10~ 3 to under 4 x 10 ~ 3 as the 

 temperature is raised to 500 c C, the conductivity rising 

 again to about 4*7 x 10~ 3 on cooling. 



As regards basalt, the second series of determinations was 

 so much more satisfactory than the first, with regard to both 

 the conditions of the experiment and the consistency of the 

 results, that there seems to be little reason to question it, 

 especially as the value obtained in it for the specific heat 

 agrees so well with the known value at low temperatures. 

 Further evidence of the different effect of rise of tempera- 

 ture on the two rocks is given by the disk experiment, which 

 supports the later results. According to these the con- 

 ductivity of basalt rises slightly with the temperature up to 

 about 270° C, above which it is constant and nearly equal to- 

 4 x 10~ 3 up to 600° C. There is also, however, a small initial 

 permanent lowering of conductivity caused by heating the 

 rock. 



This difference in the behaviour of the two rocks may 

 possibly be explained as follows : — The lowering of the con- 

 ductivity of granite is- chiefly, or perhaps entirely, due to the 

 formation of very minute cracks caused by the unequal 

 expansion of the different mineral constituents. These 

 cracks, which are formed as the rock is being heated, will 

 naturally tend to close up again as the rock cools, thus 

 causing a partial recovery of conductivity. Basalt, on the 

 other hand, being much finer grained does not show this 

 effect to nearly the same extent, though the small permanent 

 fall of conductivity noticed is probably due to the same 

 cause. The small rise of conductivit}^ caused by heating- 

 basalt to about 250° 0. may be due to another much coarser 

 system of cracks, which were actually visible as very fine 

 lines on the surfaces of all the specimens of basalt used. 

 These cracks were almost certainly formed during the 

 orioinal cooling and contraction of the rock, and hence 

 would partially close when the rock was expanded by heat, 

 thus raising the conductivity. These coarser cracks were 

 not noticed in the granite specimens, probably owing to 

 the different mode of formation of that rock. Their absence 

 probably explains why the low temperature conductivity of 



