ON THE THERMAL CONDUCTIVITIES OP CERTAIN ROCKS. 25 



ascribed to the numberless small precautions required to ensure perfect 

 accuracy, than to any constant errors of the methods with which either of the 

 two series of determinations is now believed to be aifected. 



In concluding this description, some remarks on the results of the new 

 experiments that have been carried out will serve to show what new data 

 have been obtained, and how far the observations made last year are corro- 

 borated and confirmed by the slightly modified apparatus and method of 

 procedure that has been adopted to extend the series. 



Among the points of principal importance noticed last year the following 

 facts of great interest already ascertained have now been verified and con- 

 firmed. Quartz is still found to have about the same high thermal conduc- 

 tivity (85-88 concisely expressed, as explained at the beginning of this 

 Repoi-t) compared to the other rocks which had been pre^^l0usly observed. 

 The direction in which heat is transmitted through slate is a very important 

 condition in regard to its conductiug-power — the conductivity of good Welsh 

 (Festiniog) slate cut across the cleavage being, however, to that of a plate of 

 the same stone cat parallel to the cleavage-planes, as 5 : 3 from this year's 

 experiments, instead of 6 : 3 very nearly, as observed in the same slate 

 specimens last year. The notable part of this difference of the two years' 

 observations is in the better-conducting cross-cut plate (54 instead of 66), 

 although the other Icss-conductuig plate (31-5 and 3:2-5) has nearly the same 

 conductivity as it appeared to have last year. Cleavage-fractures which the 

 cross-cut plate has suffered, and their repairs, rendering its surfaces uneven 

 and the water-junction contacts consequeutlj^ somewhat imperfect, have 

 probably caused this apparent loss of conductivity in the transverselj' cut 

 specimen of slate. But this latter still exhibits a much higher thermal con- 

 ductivity than that shown by the plate from the same piece of slate cut 

 parallel to its cleavage-planes. A less distinct difference was found this 

 year in similarly sawn and tested plates of clay-slate cut across and parallel 

 to the planes of cleavage or of foliation ; biit the stronger kinds of the stone 

 which supplied a transverse section (as well as the less fragile plates cut 

 parallel to the planes of cleavage) presented the appearance of cleavage and 

 foliation only very imperfectly, and much less remarkably than the specimens 

 of ordinary slate from Wales. The thermal conductivity of the soft clay- . 

 slate is also less in all directions (26-28-5) than the least observed conduc- 

 tivity (31-5) of Welsh slate cut parallel to its cleavage-planes. 



The observations of the effect of moisture in increasing the conductivity 

 of the porous rocks, when thoroughly saturated with water, entirely corro- 

 borate the similar observations made last year. When the great pressure 

 required to force a sensiblequantity of water through such rocks as sandstone 

 and others which were tested is compared with the verj' feeble currents 

 which differences of temperature and of density of the water in their cavities 

 can produce, it appears evident that the very marked increase of conductivity 

 observed in such cases cannot be owing to convection- or gravitation- 

 currents in the water which the saturated rocks contain, although the 

 mobility of the liquid by diffusion and consequent intermixture of its mole- 

 cules probably assists the direct condueting-power of water in the transmission 

 of the heat ; and the resulting conductivity of water, free from the action of 

 convection-currents, appears to be at least equal to that of some rock-species 

 whose thermal conductivities are either the last or nearly the lowest in the 

 present list. 



The thermal conductivities of certain new species of rocks are now also 

 assigned, the values of which, although they are few in number, appear to 

 possess considerable interest from a mineralogical as well as from a geological 



