164 



Prof. J. Prestwicli. 



The received conductivity of chalk, sand, clay, and some sandstones, 

 through which so many of the bore-holes and shafts in which the 

 observations have been made have been carried, is low. It has been 

 inferred in consequence that the isothermal lines of depth are closer 

 together in these rocks than they would be in the underlying crystal- 

 line and igneous rocks, and therefore that these surface rocks give 

 us a more rapid rate of increase than would be found to obtain at 

 greater depths. But it must be remembered that in general the expe- 

 riments on the conductivity of rocks have been made with specimens 

 carefully dried ; whereas all underground rocks hold a certain 

 quantity of water, the water of imbibition ; and further below a 

 certain level — that of the line of water-level — they are charged w/ith a 

 further quantity of water, or the water of saturation. Every bore- 

 hole or shaft has to do with rocks under these conditions. Now, the 

 experiments of Messrs. Herschel and Lebour show that, there is a 

 remarkable difference between the conductivity of rocks in their dry 

 and wet state. Thus, taking the rocks just named, these differences 

 are as under, Jc being the thermal conductivity, and r the thermal 

 resistance. 





Dry. 





Wet. 







t C ~ 



h. 



r. 



t 



Jc. 



r. 





0-00105 



952 



0-00820 



122 



Clay 



0-00250 



400 



0-00370 



270 



New Red Sandstone. . 



0-00250 



400 



0-00600 



166? 



It appears by this that sand, which is one of the worst conductors 

 when dry, becomes one of the best when wet, being only exceeded by 

 rock salt, vein quartz, and qaartzite ; while the average conductivity 

 of the three substances above named exceeds when wet (&=0'00597, 

 r = 186) that of many igneous rocks. The average of five common 

 varieties of the latter gives £=0-0524 and r=196 ; with five varieties 

 of granite the average is &=0'00584 and r=173, and taking 

 together several varieties of Palaeozoic limestones A; = - 00572 and 

 r=177. 



Experiments on wet chalk, oolite, and coal-measure sandstones are 

 wanting, but as the water of imbibition of these several rocks is as 

 under, it is to be inferred that when wet their conductivity would be 

 raised in proportion with that of the others : — 



Per cent, 



of water. Per cent. 



Coal shale 2*85 Chalk 24'10 



Sandstone . . 4'37 to 1315 Cal. freestone. 16'25 



On the other hand, the harder crystalline and igneous rocks imbibe 

 but little water : — 



