58 EEPORT — 1875. 



method of comparison appears to be the best adapted, by its directness and sim- 

 plicity, for determining relative resistances, the conditions of contact and of 

 communication of heat across the junctions appear to be subject to so much 

 variation that results of great discordance only have hitherto been obtained. 

 Before abandoning a process which recommends itself by its ease and sim- 

 plicity, further experiments, however, will be tried to remove if possible, 

 from a method which promises in the sequel to become so much more expe- 

 ditious, all the most influential sources of disturbance. 



The unit in which the absolute conductivities are expressed is the same as 

 that adopted (employing the centimetre, gramme, and second as its basis) in 

 the Table of the last Report ; and the absolute resistances in the fourth column 

 of the Table are the simple reciprocals of these, or the quotient of unity 

 divided by the absolute conductivities. As it is in the form of this quotient 

 or of the absolute resistances that the capacities of rock-strata for conducting 

 heat are most conveniently employed in calculation, a graphical construction, 

 and at the same time a convenient symbolic scale of the various grades or 

 degrees of absolute resistance presented by different species of rocks, is 

 here annexed (p. 59), with a view of exhibiting to the eye the general extent 

 and character of their specific variations. 



Among the rock-sections re-examined, three only (Kenton sandstone, 

 Calton trap, and English alabaster) are displaced, in the present list, from 

 iheir previous order of succession. The rough and porous surface of the 

 Kenton sandstone placing it more effectually in contact with the moist 

 cement than the smooth surfaces of other rocks, may be accepted as an 

 explanation of the high conductivity which it now presents ; but the rough 

 surface of the Calton trap and the smooth faces of the alabaster (although 

 these rocks are not, like the specimen of sandstone, extremely porous) would 

 give rise to the same or to an opposite variation ; while both are lower in 

 the list than the smooth-faced red serpentine, which has not altered its 

 position. The defective indications of the first constructed thermopile may 

 have introduced errors of the determinations in these latter cases, whUe it 

 may probably be to its porosity alone that the Kenton sandstone now owes 

 its somewhat superior position. 



The question of the effect of porosity and of the saturation of rocks with 

 water in increasing their conducting-power was suggested by the late 

 Mr. W. J. Henwood as oije deserving of the Committee's accurate investi- 

 gation ; and several observations for this purpose were made, of which the 

 resiilts are included in the Table. It was found that of three porous rocks 

 examined, Kenton sandstone absorbed (when freed from air in vacuo) 5"7 per 

 cent, of its weight of water, while its conductivity rose in consequence from 

 549 to 594, or 8 per cent. A plate of fine red buUding-brick, whose absorp- 

 tion of water was lo'6 per cent, of its weight, received by this treatment an 

 increase of conductivity from 147 to 247, or 68 per cent. ; whUe a plate of 

 plaster of Paris which absorbed 26 per cent, of its weight of water, rose in its 

 conductivity by the saturation from 120 to 160, or not more than about 

 33 per cent. There is no appearance of regularity in these increases, but to 

 a more copious saturation with water it is evident that a higher proportion 

 of increase of the conductivity is connected ; and again, comparing the highly 

 conducting sandstone and the badly conducting brick or plaster, it is also 

 evident that to every percentage weight or measure of water absorbed by 

 the badly conducting substances, there corresponds a considerably greater 

 percentage increase of the conductivity than for the absorption of the same 

 percentage quantity of water by the better conductor. That this may arise 



