324 REPORT — 1873. 



supposed tliat by including the rock to be tested between the temperature of boiling 

 water on one side, and that of spi-ing-cold water in the thermometer Hask on the 

 other side, the required conditions of a known difference of temperature would be 

 attained, while the rate of ascent of the thermometer in the colder vessel at the 

 same time marks the quantity of heat transmitted. But so far are the two surfaces 

 of the rock specimens from taking up the temperatures of the metal plates with 

 which they are in contact, that, with the rough means of determining their real tem- 

 peratures which were first employed, »o sensible difference xchatever could be observed 

 between them ! The small difference which without doubt exists is suiEcient to 

 ' transmit the small quantity of heat which passes, and the whole rock plate assumes 

 very nearly the mean degree of temperature between that of the boiler on one side, 

 and of the" cold-water flask on its other side. In this state of uncertainty regarding 

 the effective difference of temperature, it is quite obvious that no conclusions of the 

 nature of a numerical comparison can be made between the various rock sections ; 

 but a trial of each was yet made in the apparatus in order to determine the rate of 

 flow of the transmitted heat. 



Out of six specimens thus tried, slate plates cut parallel to the plane of cleavage 

 transmitted the heat faster than any of the others. When the flow of heat had 

 become uniform, the water was raised I'' F. in thirty- two seconds. With marble, 

 sandstone, granite, and serpentine, about thirty-nine seconds were required to raise 

 it by the same amount. The greatest resistance to the passage of heat was offered 

 by two specimens of shale (grey and black) from the Coal-measiu-es in the neigh- 

 bourhood of Newcastle, which occupied forty-eight or fifty seconds in raising the 

 water one degree, ot half as long again as the time taken by the plate of slate. 

 The black shale is highly fossiliferous, and it allows heat to pass more slowly 

 than the other harder and more compact grey species of the same kind of 

 rock. 



These experiments were not extended further, as uncertainty regarding the real 

 temperatures to which the surfaces of the plates were exposed introduced an im- . 

 known element into the question of their conducting-powers. Some experiments, 

 however, were made, which makes it probable that this difficulty can be removed. 

 It was found that the flow of heat is very little diminished by lifting the slabs of 

 rock off the heating plate, and also separating them to various distances from the 

 thermometric flask by introducing felt wads of a few different thicknesses between 

 the surfaces. A film of air (as already observed by Peclet, or of water if steam or 

 water is used to heat the plates) adheres to and protects their surfaces by its bad 

 conducting-power from becoming hot or cold, and thus opposes a certain resistance 

 to the passage of the heat. It is not improbable that the resistance thus produced 

 is the same for fi-esh cut and smoothly ground surfsices of all the different kinds of 

 rock ; and by using difl'erent thicknesses of one of them its amoimt might be deter- 

 mined and employed as a correction in estimating the conducting-powers of all the 

 other kinds of rock subjected to the trials. Although the results of this method 

 would certainly be of the greatest interest in connexion with man}' practical con- 

 trivances for transmitting heat from liquid or gaseous to solid bodies, and the 

 reverse, yet a less circuitous method, as afibrding the desired results more speedily 

 to present them to tlie Brilish Association, seemed to be preferable, and the follow- 

 ing direct observations were therefore adopted in their stead. 



A slender iron wire was joined at its two ends by twisting them on to two pieces 

 of similar platinum wire, which were coniiected by long copper wires with the 

 terminals of a Thomson's reflecting galvanometer provided Avith a millimetre scale. 

 Wlien the two platinum and iron j unctions were warmed to different degrees, the 

 galvanometer showed the difference between their temperatures on its scale. The 

 twisted junctions were fastened on the tops of two small corks, so that they could 

 be pressed against the surfaces of the rock ; and in one arrangement the corks were 

 attached to the heating and cooling plates of the heat-apparatus, and the thermo- 

 electric couples were thus supported by the corks so as to touch the rocks. In this 

 position they recorded the state of temperature of the plate of stone ('/( situ, Avhile 

 the heat conducted through it was at the same time being measured by the ther- 

 mometer. The divisions of the galvanometer scale were themselves estimated in 



