20 REPORT— 1876. 



thermal resistances observed during the past year and communicated in this 

 Report may be exhibited with equal clearness, and an easy comparison may 

 by this means be made of the values found in this and last year's series of 

 experiments where the same rock-specimens, or specimens of very closely 

 allied kinds of rock, were submitted in the former and in this year's series 

 to examination. A slight change, however, is here introduced in briefly 

 describing the results obtained numerically, by employing, instead of the sig- 

 nificant figures of those results (as was done in the last Report), the tenth 

 part of them as a brief expression for the absolute thermal conductivity. 

 Thus the absolute thermal conductivity of galena in the present list being 

 0-00705 in centimetre-gramme-second units, hitherto described for brevity by 

 its significant figures 705, will be spoken of in this Report as 70-5, to which 

 the meaning may conveniently be attached that 70"5 gramme-degree units of 

 heat per second pass through a plate of galena one centimetre thick, having 

 an area of one square metre, for a temperature-difference of one degree be- 

 tween its faces. 



The method of investigation without the use of a thermopile has hitherto 

 proved unsuccessful, no soft material capable of effecting a close junction 

 with the rocks having yet been found of sufficiently constant resistance to 

 afford a useful standard of comparison with them when the rocks are intro- 

 duced between its layers ; but the progress of the investigation has shown 

 that a simple water-film (if it could be preserved from drying off with porous 

 rocks) effects a complete junction between them and any impervious surface, 

 as that of caoutchouc, against which they are pressed. A similar film of oil, 

 it appears from some experiments recorded in the presscnt list, is less effective 

 for the purpose ; and to ensure a constant water-film in which the thin wires 

 of the thermopile could be placed, pieces of well-soaked bladder kept soft in 

 water rendered antiseptic with carbolic acid were laid on the india-rubber 

 faces of the boiler and cooler, so as to press the thermopile-wires against the 

 rock with a constantly moist and uniformly wet surface. The duration of an 

 experiment and the temperature to which they were exposed (usually be- 

 tween 100° and 120° l^'.) were never so great as to cause the bladders to 

 approach dryness before the termination of the experiment. The proportion 

 of moisture absorbed by the rocks (when sensibly porous) was ascertained, 

 and it was always such a small fraction of that imbibed by the same rocks 

 thoroughly soaked in vacuo that it i^robably exercised a scarcely sensible 

 influence on the results. Its amount, and that of the full quantity of water 

 absorbable by the porous rocks tested, is stated in the list; and from the 

 corresponding alteration of the observed conductivity some idea of the pro- 

 bable correction necessary to be applied for the presence of moisture in some 

 of the porous rocks during the process of the experiment may be obtained. 



The two chief defects of the thermopiles used hitherto had been their 

 thickness (making them intrude too far from the rock-surface into the badly 

 conducting strata with which it is in contact), and the false thermoelectric 

 currents proceeding from irregularities of material and internal condition of 

 the wires subjected to great varieties of temperature along their length. To 

 diminish the former source of error, Avires less than half a millimetre (040 

 millim., or -^ inch) in .diameter were used and neatly soldered at the 

 junctions ; and to counteract as far as possible the remaining evil, they were 

 chosen of the most dissimilar metals (iron and German sUver), and twelve 

 junctions above and twelve below the rock-plate formed a continuous circuit 

 giving a very strong thermoelectric current. The whole resistance of the 

 circuit (including the 20 ohms usually added to bring its indications con- 

 veniently within the scale of a Thomson's reflecting galvanometer) was 40 



