ON THE THERMAL CONDUCTIVITIES OF CERTAIN BOCKS. 133 



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

 M.A., F.R.A.S., and Gr. A. Leboub, F.G.S., on Experiments^ to 

 determine the Thermal Conductivities of certain Rocks, showing 

 especially the Geological Aspects of the Investigation. , 



The best means used for determining the thermal conductivities of sub- 

 stances are of two distinct descriptions, which may be denoted severally 

 as direct and indirect methods of procedure. In methods of the former 

 kind which were principally used by Peclet, and to which this Committee 

 has had recourse, the rate of passage of heat through the trial substance 

 is measured by the change of temperature of a standard body which it 

 leaves or enters, while the temperature of the trial substance itself is 

 practically free from alteration. In methods of the indirect kind the 

 measure of the quantity of heat which passes is given by changes of 

 temperature of the trial substance itself, and a knowledge of the thermal 

 capacity of the substance per unit of volume is therefore necessary for 

 their application. These latter methods commend themselves not only 

 by a larger choice of conditions most favourable for exact observation 

 which they offer, but also by the absence of any discontinuity in the 

 materials through which the passage of the heat takes place, and conse- 

 quently of any uncertainty about the area of surface which enters into 

 conducting contact where two solid bodies are placed or pressed together 

 as perfectly as possible. The Committee has indeed found that a film of 

 water wetting two such surfaces completely, and uniting them, renders 

 the whole of their areas effective in conducting heat without introducing 

 any sensible resistance, and fine wires of a thermopile lodged in this 

 water stratum measure the two extreme temperatures of a trial layer of 

 the substance to be tested with the greatest certainty and convenience ; 

 but the application of water to porous and to soluble bodies like chalk 

 and rock-salt gives for obvious reasons doubtful values of their conduc- 

 tivities, and only those bodies which resist, and which do not absorb 

 water can be tested accurately for thermal conductivity by those means.* 

 It has therefore appeared very desirable to the Committee, for the suc- 

 cessful use of processes in which the indirect method is adopted, to deter- 

 mine as exactly as possible the specific gravities and specific heats of the 

 series of rock-specimens now at their disposal ; and in the table presented 

 with this report the results of their observations of these quantities are 

 given of every plate which has hitherto been prepared for their examination. 

 The measurements have been made at their request by Mr. J. T. Dunn,f 

 whose name as a careful and active prosecutor of these investigations the 

 Committee desires, in the event of its reappointment for another year, 

 to add to the rather restricted number of its present members. A brief 

 description of the process of these observations will explain the results 

 of the measurements obtained from them, the statements of which, in 

 a very abridged form, are presented in the table. 



* As a means of excluding water without breach of its intimate contact with 

 soluble or porous rock surfaces, it has been suggested to the Committee to " silver " 

 the plate-surfaces with mercury and tinfoil in the same manner that the face of a 

 plate-glass mirror is silvered ; and there appears every reason to expect that the coat- 

 ing of amalgam thus applied, while impervious to water would firmly attach itself to 

 and fill up quite solidly all the asperities of the surface ; but the actual efficacy of 

 this method for some of the very porous rocks has not yet been practically tested. 



f B. Sc, Demonstrator of Chemistry and Physics in the University of Durham, 

 College of Physical Science, Newcastle-upon-Tyne. 



