ON THE CONDUCTIVE POWEES OF VAEIOUS SUBSTANCES. 
821 
as varying from 2 '5 to 3'0 ; and very hard and compact Mountain Limestone may be 
estimated at 5‘5. These, according to my results, may be taken as the approximate 
limits for any rocks containing a sufficient quantity of lime to be designated as cal- 
careous rocks. 
Arenaceous and siliceous rocks have a somewhat wider range of conductivity. That 
of friable New Eed Sandstone was about ‘25, while that of very hard Millstone Grit rises 
to as much as ‘75. With respect to argillaceous rocks, my experiments have been less 
complete. They show, however, that when not indurated, and in that state of dryness 
or moisture in which they may be supposed generally to exist in mass, their conductive 
power may be taken at about *3. I have not tried any hard clay rock, but, judging from 
other cases, I should expect it to rise to about ’6. 
The igneous rocks have uniformly high conductive powers. In ten cases they varied 
from ’53 to 1, the mean of them being '72. In the old sedimentary rocks (Cambrian 
or Silurian), not calcareous, they appear to vary from about ’5 to ’G, or probably still 
higher. 
It is manifest that whatever may be the mineral constituents of a rock, its conductive 
power depends very much on its degree of hardness and compactness, that kind of indu- 
ration which results from some chemical action ; while that compactness and comparative 
hardness which arise merely from mechanical pressure, have less influence than might 
have been anticipated. In such substances as wax and spermaceti, it is not appreciable 
in my experiments ; nor does it appear to be so in pure calcareous masses like chalk. 
In clay, however, the effect is very sensible. Dry sand is not sufficiently adhesive to be 
formed into blocks convenient for the purpose of experiment, even under very heavy 
pressure. Pressrue would probably have little effect on the conductivity. 
Temperature has a considerable effect on the conductivity, when it approaches the 
temperature of fusion. Biot showed this to be the case with certain alloys which are 
easily fusible ; and I have also found the same true for wax and spermaceti. There 
seems no reason, however, to suppose that temperatures differing sufficiently from those 
of fusion, produce much effect on the conductivity. 
The conduction of heat is very sensibly affected by discontinuity in the conducting 
mass. I have showm, however, that in the case of a sandstone of about a mean conduct- 
ing power, a division of the mass into beds of one foot in average thickness would not 
diminish the conductive power by more than about one-twentieth part. 
In rocks which are great absorbents of water, the conductivity is much affected by 
the degree of moisture. The effect increases with the quantity of moisture up to a 
certain point, which, however, appears to be considerably short of saturation. It would 
appear slightly to decrease again. In chalk, friable sandstone, and clay, the conductive 
power is increased in a considerable ratio. Hard sandstone, and any highly indurated 
rocks, are comparatively bad absorbents, and their conductivity appears to be little 
affected by any moisture they are capable of imbibing. 
MDCCCLVII. 5 p 
