OM THE THERMAL CONDUCTIVITIES OF CERTAIN ROCKS. 1)7 



and sand were saturated with it after their conductivities in a dry state had 

 been tested, the soft materials being placed for this purpose in one of the 

 thick caoutchouc belts, which was closed at the top and bottom with sheets 

 of thin paper and gutta-percha tissuo (used to keep water of the thermopile 

 lappets from the dry saud, and roughened to make it touch the sand per- 

 fectly inside, like sandpaper, with a coat of sand attached to it with shell- 

 lac). The water was added to the sand in its cell (by a pipette introduced 

 below, until it overflowed from an opening in the top), so as to effect its 

 thorough saturation ; whilst the pressure (of 60 lbs. or 80 lbs.) upon it in 

 the apparatus would prevent any water from remaining in the cell, except 

 saeh as was contained in the interstices of the sand. The low conductivities 

 of dry sand and of dry clay are (for their weights) remarkable; and tho 

 effect of adding water to them is to increase their low conductivities very 

 considerably. But the effect is much more evident in sand than in wet 

 clay, where the possibility of any convection-currents of the enclosed water 

 assisting the heat-transfer is effectually excluded ; in the interstices of the 

 sand, on the contrary, however feeble they must be, it is yet possible that 

 the}' may materially assist the process of transmission of the heat. Jt indeed 

 appears probable that the fluid freedom of the water in the interspaces which 

 it fills in the sand may in this case enable gravity to have some share in 

 carrying through the open channels some ascending currents of warm water 

 and some descending currents of cold water, in spite of the effects of friction. 

 If this explanation can bo conceded, it may be fairly granted from the great 

 increase of conductivity imparted to loose sand by water in its passages, 

 which will be noticed in the Table, not less than what is found in some 

 rocks notable for their good conducting qualities, that tho presence of the 

 water used to saturate the lappets of the thermopile, easily percolating their 

 silk tissues, must place the thermoscopic wires as thoroughly in contact with 

 the rock as if they were cemented to it with a thin film of rock at least as 

 good in its capacities of conduction as some of the best conductors of the 

 ordinary varieties of rocks which have been examined. The Committee has 

 recorded this observation of the good conducting-powcr of water contained in 

 such a vehicle as sand or other freely permeable substance with considerable 

 satisfaction, from the renewed confidence which it enables it to place in 

 the method of experimenting which was adopted, and from the fresh assur- 

 ance that it gives of the correctness of the results which have thereby been 

 obtained. 



In presenting the results arrived at by these means of experiment during 

 the past year, the Committee feel very certainly persuaded that the lists of 

 absolute thermal conductivities and resistances which accompany this Report, 

 and which are appended in three previous Tables to the earlier Reports of 

 the Committee during the last three years, arc near approximations (although 

 they differ by fluctuating and insidious faults of observation from a constant 

 mean) to those expressions for the thermal conductivities of the most im- 

 portant and abundant kinds of rocks which it has been one of the Committee':- 

 principal objects in the present investigation, by the best and in oat conclusive 

 possible processes of experiment, to ascertain. 



1S77. H 



