660 PROCEEDINGS OF THE AMERICAN ACADEMY. 



white marble from different quarries may differ as much as thirty per 

 cent. In the table I have, in two or three cases, taken the average of 

 nearly agreeing results. 



It is evident that the conductivities of the specimens of dry rock which 

 I have examined are relatively rather low. 



The Jefferson Physical Laboratory, 

 April, 1903. 



