Mr. Hopkins on the Conductive Powers of various Substances. 313 



was there any sensible effect with chalk between a pressure of 

 4300 lbs. and 7500 lbs. per square inch. 



Clay which when iuconipressed had a conducting power ='2(», had 

 when compressed with 7500 lbs. per inch, a power ='33; and the 

 conducting power of a mixture of sand and clay in equal quantities 

 rose from -SG to "378 by an increase of pressure from 4300 lbs. to 

 7500 lbs. per inch. 



Generally the effect of jiressure is much less than might have 

 been anticipated. 



Effect of Biscontinuity. 



4. When the conducting mass consists of a number of strata 

 superimposed on each other, the mathematical problem presented to 

 us requires a distinct investigation, wliich is here given under a very 

 general form, together with the experiments necessary to determine 

 the effect of this kind of discontinuity. The result is that if a mass 

 of sandstone consisted of a number of strata, the conducting powers 

 of which should be about '5, the mean conductivity of the whole 

 would not be diminished by more than about -v\,th part, supposing the 

 average thickness of strata to be 1 foot ; or by about Yijth, if that 

 average thickness should be 6 inches. This effect is much less than 

 might possibly have been anticipated. 



Effect of Moisture. 



5. This effect was very considerable in those rocks which are great 

 absorbents of water. The maximum effect appears to be produced 

 by a quantity of moisture which falls considerably short of producing 

 complete saturation. The conducting power of a piece of dried 

 chalk was =• 1 9, but became ='30 when the substance was very 

 moist. That of a well-dried piece of new red sandstone was ="25, 

 but became as much as '60 when saturated. Both these substances 

 absorbed a large quantity of water. Ancaster oolites absorbed con- 

 siderably less, and their conductivity was affected in a smaller degree. 

 For a block of dry clay the conductive jjower was "23, and became 

 •37 when well moistened. Close indurated sandstone, palaeozoic 

 rocks of close texture, and igneous rocks are bad absorbents, and are 

 very little affected in their conductive powers by moisture. 



Comparison of Deductions from Theories of Terrestrial Temperature 

 ivith the Results of Observation. 



G. It has long been established by mathematical investigation, that 

 if a large globe like the Earth be heated in any manner and in any 

 degree, its temperature at points not too remote from its surface, and 

 after a sufficient lapse of time, will necessarily become such that the 

 increase of temperature in descending along a vertical line will be 

 proportional to the increase of depth. In this enunciation, liowcver, 

 it is assumed that tlie conductive power throughout the mass, or at 

 least throughout its more external portion, is uniform. Tiic differ- 

 ence of conductive power between the unstratified and sedimentary 



