96 report — 1877. 



turbing causes ; and the best endeavours of the Committee have therefore been 

 used to obviate and to diminish the effects of their action as much as possible. 

 This they have, they believe, accomplished in the main successfully ; but 

 instances yet frequently occur which show that without special and very 

 close attention to them the most unequivocal experiment, apparently, may 

 yet mislead ; and it is not without this recognition and probable explanation 

 of some of the obvious discrepancies in the accompanying Table that they 

 venture to produce the values which it contains as probably exhibiting very 

 approximately the true absolute thermal conductivities of the various rock- 

 specimens which they have tested. 



The quartzites (compact siliceous rocks) from Schiehallion agree in their 

 conductivities with crystalline and opaque white quartz. Another class of 

 rocks from the neighbourhood of Schiehallion experimented on is the class 

 of micaceous sandstones, or flagstones, which cover a large area in the in- 

 terior of Scotland. The mica, which is abundant in these sandstones, appears 

 to have imparted to them a slaty cleavage, the plane of which in their actual 

 positions is seldom horizontal, and is more often 30° or 40° inclined to the 

 horizon. Like other sandstones they may be readily broken across, as well 

 as in the direction of their cleavage-planes, and no difficulty occurred in 

 obtaining some trial sections of them in these different directions. The 

 result shows that the conductivity increases (on the average of the samples 

 tried ) continuously from that of heat transmission across to that of its trans- 

 mission along the direction of the cleavage-planes in the proportion of 

 2 : 3, not quite so great as that observed in slate *, apparently from the less 

 perfect ease and liability to cleavage which these stones present. A kind of 

 firestone kindly supplied to the Committee by Mr. Baldwin Latham, C.E., 

 from quarries at Godstone, in Surrey, where it is largely extracted on account 

 of its unalterable qualities under the action of certain furnace-heats, which 

 exhibits very regular bedding in the quarries, but of which the cleavage is 

 yet either insensible or exceedingly imperfect, exhibits no signs of increase 

 of conductivity for heat-transmission in the direction of the bedding-planes. 

 But some specimens of altered shale compacted apparently to perfect uni- 

 formity and hardness by contiguity to the once molten intruder of whin 

 rock under which they lay, so as to break with the same facility in all 

 directions excepting where shrinkage-cracks in and across its plane of bed- 

 ding (apparently like those in whin rocks) have parted it by the heat to 

 which it lias been exposed, still exhibits a tendency of heat to traverse it 

 more freely in the direction of its original bedding-planes than in the trans- 

 verse direction, in the proportion of about 3 : 4 1. The conductivity is, at the 

 same time, raised considerably by the semifusion of the materials above 

 that of ordinary shales, of which some new trials which were made this 

 year aro also included in the present Table. 



The specimens of granite, of porphyritic trap rock, and of mica schist 

 obtained from Loch Bannoch in Perthshire, present conductivities which 

 resemble very nearly that of the grey Aberdeen granito with which they are 

 here compared ; and some new trials of varieties of limestone, chalk, and 

 marble have been made, which maybe regarded as in satisfactory accordance 

 with what have been previously observed. 



In order to establish and confirm the good conductivity of water which 

 was revealed in some of the experiments made with it last year, dry clay 



* About 3 : 5. See these Reports, vol. for 1870, p. 24 



t The experiment having been made on two specimens only, which were not cut from 

 the same block, very great weight cannot bo claimed at present for this preliminary trial. 



