ON THE TIIEIiJIAL CONDUCTIVITIES Ol' CERTAIN ROCKS. 



19 



Ifou^h comparison of coefficients with IJatthiessen's. — It is perhaps worth 

 ■while to give the following rough comparison between the results for the 

 A ariation-coefficients which we have obtained in the neighbourhood of 10^ C. 

 with the mean results of Matthiessen. 



There is no striking difference except in the case of Pt Ir, where the 

 alloy of which the coil is made must approach much nearer a pure metal 

 than Matthiesseu's alloy (33-4 per cent, iridium) did. 



Biscrepaney in Coil G tvith former measurements. — The only other point to 

 which we have to call attention is the discrepancj^ between former and 

 present measurements in the coil G, whose resistance seems to have gone 

 down since it was last tested. 



In conclusion we venture to suggest two alterations in the construction of 

 standard coils, which, as far as our experience goes, would be improvements. 



First, to make them flat instead of cylindrical. This would facilitate 

 stirring when the coils are immersed in any liquid. 



Secondly, to insert as near the wire as possible a properly insulated junction 

 of a thermoelectric couple, the other junction of which should be fastened on 

 the outer case of the coil. Several of these fitted to each coil would do away 

 with a great deal of the trouble and uncertainty attending the temperature 

 determinations required in comparing and copying standards. 



Third Rejiort of a Committee, consisting of Prof. A. S. HerscheLj 

 B.A., F.R.A.S., and G. A. Lebour, F.G.S., on Experiments to 

 determine the Thermal Conductivities of certain Rocks, showing 

 especially the Geological Aspects of the Investigation, 



The object originally proposed by the Committee was to arrange and classify 

 the most commonly occurring rocks experimentally according to their powers 

 of conducting heat ; and it has hitherto been so far successfully attained that 

 the thermal conducti\dties of an extensive series of ordinarily occurring rocks 

 have been shown to differ from each other on a very strongly marked scale of 

 gradation, which it was endeavoured to represent graphically in the Com- 

 mittee's last Report by a series of ascending steps of absolute thermal resist- 

 ance, or resistance to the passage of heat offered by the different rocks. To 

 every 200 imits of this ascending scale a new letter of the alphabet, starting 

 ■with A for the interval 0-200 of absolute resistance, was assigned ; the values 

 of the resistances were shown graphically, and the various rocks that arrange 

 themselves under the several classes so formed could be readily discerned. 

 By adopting this graphical mode of representation the values of certain 



c2 



