ON THE THERMAL CONDUCTIVITIES OF CERTAIN ROCKS. 59 



Table, with the probable values which they may be conjectured to indicate 

 very nearly as concluded from comparisons of the known with the defec- 

 tively observed conductivities of the twb rock-plates of reference. 



The uncertainty which attached, in last year's Report, to the obser- 

 vations of the specific heats of some porous rocks, has now been removed 

 by repeating the experiment of boiling them in water, and immediately 

 weighing them to determine the quantity of boiling-hot water which they 

 absorbed. The result is that the assumption made in last year's Report 

 that the quantity of water so imbibed is in general half the weight which 

 they are found to have gained by an immediate immersion, after boiling, 

 in cold water, is fully verified. The fraction of the total water-gain which, 

 for example, entered the six specimens of Craigleith sandstone during the 

 first process of boiling them, had the real values 0'41-0"49 (average 0"45). 

 The corrections which the specific heats of these porous sandstones given 

 in the Table of last year's Report require for this little imperfection of the 

 adopted allowance is so small as only to affect by a single significant unit 



(and in the ratio - by one or two units), a few of the thirty numbers 

 c 



given for these sandstones in the Table. The same substantiation of the 

 figures in the Table has been found for all the sandstones (water-absorp- 

 tion 6 - l-8 - 4 per cent.), and other rocks (including Mansfeld limestone, 

 absorption 8"1 per cent.), not exceeding them in porosity. Newcastle 

 firebrick (absorption 14 - 3 per cent.) is an extreme case in which the 



allowance adopted, and the values of the specific heats, and of the ratio, - 



given in the Table require no sensible correction. In rocks which, like 

 the last, exceed the pure sandstones in porosity, the rule for correcting 

 the Table illustrated by examples in the last Report, to regard the allow- 

 ance adopted in the Table as too little by a half, is now proved to be sub- 

 stantially correct, the ratios for Caenstone, Great Pyramid, and Castle 

 Eden limestones, Godstone chalk, 1 firestone, and sandstone, magnesite, 

 and plaster-of-Paris, all lying between 074 and 0*85, the last of which 

 ratios is an exceptionally high proportion, for Castle Eden limestone. 



On correcting the tabular specific heats of these very porous rocks (as 

 has been done in the short recapitulation of them given below, in the 

 manner described by some examples in last year's Report), by the actual 

 fractions of total water-gain now found to have been introduced into the 

 plates by boiling them, a close agreement of the corrected values (with 

 only one exception) is produced with the common value, about 0"20, of 

 the heat- capacities of nearly all the other rocks recorded in the Table. 

 The real specific heat, by weight of plaster-of-Paris alone, agrees (as was 

 surmised correctly in last year's Report) with that of English alabaster, 

 or gypsum, and nearly also with red and green serpentine from Cornwall, 

 in being exceptionally high (026-028). If the metallic ores, galena and 

 iron pyrites, are excluded from the list, the only other examples of rocks 

 in the Table, whose specific heats differ by more than one or two significant 

 units from the common value, - 20, are the specimens of Newcastle black 

 shale (0"29), and coal of two varieties, cannel coal, and ordinary pit-coal, 



1 The specimen of pure white chalk, whose thermal properties, as partly tested, 

 have been previously described in these Keports, having yielded and crumbled last 

 year in the experiment on its specific heat, could not be submitted this year to a 

 repetition of the same experiment. 



