128 REPORT — 1881. 



to those near the cool end of the bars, where the stationary temperatures 

 shown by the thermometers inserted in the bars were, in general, very 

 little in excess of the temperature of the surrounding air. The numbers 

 in this series were divided by 0-72 to make that for quartz correspond 

 to the thermal conductivity 0-0096, found for that substance as the 

 average result of the Committee's experiments described in former years 

 in these Reports. The constant rate of logarithmic decrement in the 

 quartz-bar, in fact, shows that its conductivity was more exactly measured 

 by that quantity than in the case of the other bars whose rates of 

 logarithmic decrement were very variable. 



The processes of experiment used by Hopkins and by Less to compare 

 together the thermal conductivities of two considerable series of rocks, 

 were those of Fourier and of Peclet, with some slight modifications, and 

 they differ principally from the latter, and from that employed in the 

 experiments of these Reports, in not affording the absolute, but only 

 the relative, conductivities of the tested plates. In both mercury was 

 used to establish direct contact between the plates and the heater and 

 cooler between which they were interposed ; and the heat traversing the 

 plate from below in the former, escaped to the surrounding air from the 

 bright fluid surface of the upper covering of mercury whose temperature, 

 and that of the mercury below the plate was at the same time observed. 

 In Less's experiments the heat which passed downwards escaped to the 

 outer air from a blackened copper plate pressed (like a heated one above) 

 against the rock plate with a wet-junction of mercury. A thermopile 

 placed opposite to the radiating-plate enabled its temperature to be 

 observed. 



The numbers (relative ones) 7C3 and 769, found by Less for Italian 

 and Carrara marbles are higher than the Committee's absolute (signifi- 

 cant) numbers (57 and 61) of white marbles from Italy and Sicily ; the 

 same being in general the case throughout the two lists. , The numbers 

 of Less's list have all been diminished by one-tenth to assimilate them 

 to and incorporate them in the present general list. But no altera- 

 tion of the significant relative numbers obtained by Hopkins from his 

 experiments was found to be required, the slightly defective number, 53, 

 for statuary marble being compensated, especially in some granites and 

 hard rocks, by a little excess of the relative numbers above those obtained 

 by the Committee as absolute ones for quartz and granite. Accordingly, 

 in the general list the relative numbers given by Hopkins have been used 

 directly in a proper decimal place to give average absolute values of the 

 conductivities which he found for certain different groups of common 

 descriptions of rock tested in his experiments. 



Other observations, especially those of Peclet and Neumann, are 

 originally absolute determinations. 



Of the original memoirs from which these data were extracted, and 

 also of a long series of other papers relating to measurements of thermal 

 conductivity, which mainly comprise the past history of its experimental 

 investigation, a descriptive index and digest was drawn up for the 

 Committee by Mr. Dunn. Of this abstract, as it supplies references, and 

 further particulars of the experiments which have just been described, 

 and as it affords a condensed review of the progress of experiment on the 

 physical properties which here claim attention, the titles and substances 

 ' of the publications which have most eminently extended and advanced 

 ^'" the subject are added at the end of this Report, as an Appendix. 



