ON THE THERMAL CONDUCTIVITIES OF CERTAIN ROCKS. 03 



ii can furnish some additional support from these provisional experiments to 

 the assurance which it has already gained in former years, that the values 

 assigned to them will not be found to differ, except in some rare accidental 

 cases, more than 10, or, in the least satisfactory cases, it may ho 15 or 20 per 

 cent, from the real thermal conductivities of the rock-plates examined. The 

 heat-capacity of the cooler and the rate of loss of heat through it to the outer 

 air were reobserved, and Avcro used independently in the preparation of this 

 Table, although they differed slightly (perhaps from setting and tightening 

 of its jacket with time) from similar measures of them which were used 

 last year. 



The conductivities observed this year are, for the most part, a little higher 

 than those found formerly, perhaps by reason of the new determinations used 

 of the heat-capacity and heat-loss of the cooler : but some exceptions to this 

 rule are also found ; and it must be remembered that no allowance is made 

 in these or in the earlier results for the small quantity of heat absorbed by 

 the rock-plate itself during the progress of an experiment, nor, again, for 

 the fact that the rate of absorption of heat and of its emission to the outer 

 air by the jacketed vessel of the cooler is dependent on the rate of rise of 

 water temperature inside it. The specific heats of the materials of the 

 rocks themselves and of the apparatus are not sufficiently known to determine 

 these corrections surely ; but at the common specific heat (0-2) of a great 

 number of rocks it appears that about one thirtieth of their values may have 

 to be added to the observed conductivities for the first of these considerations. 

 Whether an additive or subtractive correction is required for the second 

 cause cannot be decided, because it is an uncertainty already occurring in 

 the determination of the heat-capacity and rate of heat-loss of the cooler, 

 the judgment necessary in assigning which must be regarded as providing 

 sufficiently for this correction. Thus a correction for heat-capacity of the 

 cooler vessel and its jacket of one tenth was added to the conductivities beforo 

 observed directly, while one eighth was added in the experiments of the 

 present year, allowing a rather longer time (of three or four minutes) for the 

 heat to penetrate the vessel and its jacket. At the 6low rate (about 1° F. 

 in the same time) that the temperature of the water rises in the cooler 

 during an experiment no higher correction than this could well be admitted ; 

 for after this length of time the loss of heat from the water depends sensibly 

 upon its escape to the outer air, and no longer perceptibly upon the absorp- 

 tion or capacity for heat of the vessel and its jacket, when an experiment is 

 made to find the amount of this heat-loss by absorption. The uncertainty 

 of this correction must therefore range between one tenth and one eighth of 

 the observed conductivities, which have been used as its extreme values in 

 different cases. The apparatus may fairly be regarded in other respects as 

 perfectly heat-tight in its connexions by the thick belts of caoutchouc which 

 surround the tested plate of rock and the meeting ends of the boiler and 

 cooler pressed against it by great pressure ; and a thick wooden table 

 (through which the padded boiler top just reaches the level of its upper 

 surface) prevents any extraneous heat from the source below it from reaching 

 the testing apparatus. The discrepancies which are observed can therefore 

 only be ascribed directly to two disturbing causes. These are, the imperfect 

 closeness of contact of the thermopile wire junctions with the tested rocks, 

 and the want of absolute freedom of the wires from extraneous currents 

 arising from other actions than those of the temperatures at their points of 

 junction. It is impossible to prevent entirely the operation of these dig- 



