ON THE THERMAL CONDUCTIVITIES OF CERTAIN ROCKS. 135 



space which the absorbed water occupies in proportion, per cent., to 

 the whole volume of the rock, or the porosity as a fraction of that volume. 

 Some special deportment of the water in the rock-pores may, however, 

 sometimes interfere with such a calculation, so as not to allow the real 

 volume of the spaces open to its penetration to be thus determined. It 

 was thus observed that the addition of water to dry sand contracts, and 

 of water to dry clay expands its volume considerably, so that the per- 

 centage gain of specific gravity of these substances (dry sand and dry 

 clay) by water saturation, is not the same as their percentage weight of 

 imbibed or appropriated water. Two numbers are given for these sub- 

 stances in the column of percentage gain, the last of which, in brackets, 

 denotes the water added, while to obtain the first or the change of the 

 specific gravity, a new measure of the altered volume of the wet com- 

 pound material made by the mixture had to be obtained. 



The several experiments of heating brick plates in steam and boiling 

 water, without afterwards immersing them in cold water, showed that a 

 single such treatment impregnated the plate with about a half, or up- 

 wards, of the whole quantity of water which the plate finally absorbs by- 

 plunging it in cold water, and although the same experiments have 

 not yet been extended to other porous rocks, a provisional assumption is 

 adopted for them all, to calculate the specific heats, that the rocks which 

 sensibly absorbed water in the process were laden with one half of it at 

 the boiling temperature, when they were plunged into the calorimeter. 

 It is not possible to say if this allowance is too great or too small in any 

 given case, but any rectification which it requires cannot at least 

 exceed (positively or negatively) the adopted value itself, and the per- 

 centage change in the calculated specific heats, which a plus or minus 

 rectification to the whole extent of the adopted allowance would introduce, 

 is subjoined in the column of the table immediately following the specific 

 heats, wherever the capacity of absorption of a rock specimen was so 

 great as to render needful a distinction between its properties in the wet 

 and dry condition. If the allowance of one half of the final water- gain, 

 supposed to be introduced by boiling, is deficient, so that any fractional 

 increase, or positive rectification of it is actually required, the same frac- 

 tion of the percentage correction given in the table must be subtracted 

 from each of the specific heats recorded as the provisional observed values 

 in the table. If the allowance was in any case too great by a known 

 part or fraction of itself, and therefore required a negative rectification to 

 the extent of such a fraction, the same fraction of the percentage correction in 

 the table is to be added to the recorded values of all the different specific heats 

 to obtain their real values. The adopted allowance is probably very near 

 the truth for the slightly porous rocks (whose coefficient of absorption is 

 less than 6 per cent.), and no corrections of the specific heats given in 

 the table are required for these, nor for any of the perfectly compact and 

 solid kinds of rock in which no sensible signs of porosity and of water 

 absorption were observed. With the increa'se of porosity, however, the 

 assumed allowance probably requires an increasing addition of perhaps 

 nearly its full value for some of the most porous ones (as chalk and 

 plaster of Paris) ; but considering the smallness of the correction itself 

 in the rocks of small and moderate absorbing powers, a common rule of 

 adding half its value to the adopted allowance, and therefore of subtract- 

 ing half the percentage amount named in the table from the recorded 

 specific heats of all the porous rocks, is one whose results will certainly 

 not deviate far (it may be five or six per cent, in the most absorbent 



