58 eepobt — 1879. 



Sixth Report of a Committee, consisting of Professor A. S. Hebschel, 

 Jf.il., F.R.A.S., Professor G. A. Leboue, F.Q.8., and Mr. J. T. 

 Dunn, B.Sc, on Experiments to determine the Thermal Con- 

 ductivities of certain Rocks, showing especially the Geological 

 Aspects of the Investigation. 



The research and correspondence necessary for the completion of an 

 historical sketch of the attempts hitherto made to determine experimentally 

 the Thermal Conductivities of various Rocks occurring widely over the 

 earth's surface, which the Committee proposed to prepare during the past 

 year, are not so far advanced at present as to allow them to be compre- 

 hended in this year's Report. The Committee hopes, by continuing its 

 enquiries for another year, with the addition to its numbers of Professors 

 W. E. Ayrton and J. Perry, of the Imperial College of Engineering in Japan, 

 who have pursued the subject practically with the greatest attention and 

 success, to carry out the object of their undertaking, so as to exhibit the 

 present state of our knowledge of the data of Thermal Conductivity, 

 needful for discussions of the conditions of the earth's temperature, which 

 have been determined by observations and experiments. 



In a paper of great practical interest in this respect, 1 published at the 

 end of the year 1876, by Professor Stefan, of Vienna, a series of experi- 

 ments is described, by which he determined very accurately the absolute 

 thermal conductivity of ordinary Ebonite. The process used being the 

 same in principle (although differing from it a little in its details) , as that 

 adopted by Professors Ayrton and Perry for determining the thermal 

 conductivity of some specimens of a kind of Japanese building-stone, 

 employs for its application Fourier's formulas, and therefore gives the 

 absolute conductivity, in the first instance, indirectly, or only in terms of 

 the heat-capacity of a cubic centimetre of the trial-substance as the unit 

 of heat- quantity, instead of in absolute heat-units. The value in absolute 

 heat-units of this thermal capacity of the substance has then to be deter- 

 mined by a subsidiary experiment. As the very trustworthy value found 

 by this otherwise convenient method affords a useful standard for com- 

 parison with other methods, that adopted by the Committee was checked, 

 during the past year, by applying it to determine directly the thermal 

 conductivity of a plate of ordinary ebonite, together with that of some 

 plates of vulcanised indiarubber, with which, by the courtesy of their 

 agent in Newcastle, Mr. W. Beer, the Committee was furnished from the 

 Silvertown Works of the Indiarubber and Guttapercha Company in 

 London. 



Some omitted measurements of rock conductivities were also made at 

 the close of the past year, with the Committee's apparatus. But owing 

 to some deterioration which it has in the meantime undergone in its con- 

 dition, they are insufficiently high, as proved by the values found for red 

 serpentine and white Sicilian marble. As the results, however, possess a 

 relative value among themselves, and also in relation to these two speci- 

 mens of which the conductivities have before been very well determined, 

 they are added to the last- mentioned observations, in the accompanying 



1 ' Sitzungsberichte ' of the Imperial Academy of Sciences of Vienna, vol. for 

 1876, part ii. ; November, 1876. 



