TRANSACTIONS OF THE SECTIONS, 223 



APPENDIX. 



Notes of some Experiments on the Gonducting-powers for Heat of certain Books, 

 ivith Bemarl-s on the Geolofjical Aspects of the Investigation. By Prof. A. S. 

 Herschei, and G. A. Lebotje, F.G.S. 



A subject of considerable interest in a physical and geological point of view, as 

 illustrating the questions of underground temperature that have recently occupied 

 the attention of a Committee of the British Association, presented itself as open to 

 much more extensive experimental investigation than perhaps, from the absence 

 of anv immediate practical applications of its results, it has hitherto been thought 

 worthy to receive. The object which the authors of this communication proposed 

 to themselves was to determine experimentally the actual conducting-powers for 

 heat of as many well-defined and commonly occurring species of geological rocks as 

 they could conveniently obtain, and submit to the test of some suitable and practical 

 method of experiment. A collection of more than twenty specimens of rocks of the 

 best-marked descriptions were for this purpose selected at the well-known Marble 

 and Stone Works at Newcastle-on-Tyne, of Messrs. Walker, Emley, and Beall, 

 who at the same time undertook to reduce the blocks (together with some addi- 

 tional materials obtained elsewhere) to a uniform size and shape, to Avhich they are 

 all gauged with the greatest care. ' The plates are circular, five inches m diameter 

 and half an inch thick-, and were thus chosen as being nearly of tiie same dimensions 

 as those employed by Peclet in his investigations of the conducting-powers of 

 various substances for heat. Considerable labour and risk, however, is incurred in 

 working plates of granite and the harder stones of such thinness ; and (as the result 

 has shown) the measurements of their heat-conducting powers would have been 

 rendered both more exact and easier had a thickness of about one inch instead of 

 half an inch been adopted for the plates. A list rf the specimens employed is 

 annexed below; and it will be seen that among rocks of very wide distribution but 

 of more friable materials, as chalk, coal, sand, or marl, and some more recent sedi- 

 mentary contributions to the earth's crust, no attempt to include them in these 

 measurements has yet been made. _ . 



The pui-pose of the present note is simply to establish from the preliminary ob- 

 servations the general bad conducting-powers of the harder rocks, and to corro- 

 borate, in the case of a few examples that were numerically reduced, the conclusions 

 of a similar description that were obtained by Peclet. _ 



Description of the Apparatus.-— In order to heat the rocks, a flat-topped circular 

 tin boiler was provided of the same diameter as the rock plates, upon which they 

 could be laid so as to be exposed on their lower side to the heat of boihng water. 

 The steam produced by the water at the bottom of the boiler rises through a central 

 tube to the top, where it circulates in a steam-space formed by a perforated dia- 

 phragm placed round the top of the tube, and it emerges from the side of the boiler 

 at the bottom of the annular space formed between the boiler and the central tube. 

 The upper part of the boiler is surrounded to about an inch in depth (the depth 

 of the steam-space) bv a thick ring of wood resting upon a projecting ledge of the 

 boiler, and protecting'it, as well as the slab of rock placed inside it upon the flat 

 lid of the boiler, from loss of heat to the surrounding air. The ring of wood pro- 

 jects above the rock so as to receive a flat-bottomed tin vessel (.shaped like a conical 

 'flask) of water, of the same diameter as the rock plate at the base, and contracting 

 at the top to a narrow neck, in which a theinuometer is inserted by a cork. "S^S hen 

 the apparatus is in use, a light packing of cotton-wool is inserted between the 

 wooden ring and its contents, to keep them more efiectually from contact with the 

 outer air. 



Mode of conducting the Experiments, and !!/iejVi2es(//^s.— The heat-conducting power 

 of a substance being measm-ed by the quantity of heat that passes through a plate 

 of it of known thickness and cross section at a given difl'erence of temperature 

 between its two faces of which the interval can be measured, it might at flrst ba 



