ON THE CONDUCTIVE POWEES OE VAEIOUS SUBSTANCES. 
807 
imposed matter. I made use of mercury for this purpose in my own experiments ; and 
thus, if c denote the radiating power of mercury, we have 
k 
c 
( 6 .) 
and c being the same, whatever may be the substance experimented on, this formula 
enables us to compare the conductive powers of different substances, or to determine 
the absolute numerical values of those powers, when c the radiating power of mercury 
is known. The determination of these comparative values of k, and not those of - 
(which have more usually been determined), has formed the object of these experimental 
investigations. 
My experiments have been made on a great variety of mineral substances, and on 
some others also : I shall reserve certain details respecting them for a subsequent part of 
this paper, giving here the general results at which I have arrived. It should be 
observed, that k being a linear quantity, like h 
^as appears from the expression for , 
its numerical value will depend on the unit of length. This unit has been assumed to 
be one foot*. 
2. Expeiiments were made for the purpose of ascertaining the conductive powers of 
calcareous, argillaceous, and siliceous masses in a state of dry powder. The first was 
obtained from a piece of pure chalk rock, the second from a piece of clay which ap- 
peared to have very little admixture of other elements, and the third was obtained from 
a piece of New Ked Sandstone. All were thoroughly dried. The results were as 
follows : — 
Values of-* 
C 
Calcareous powder -056 
Argillaceous powder *07 
Siliceous powder T5 
A mixture of the two last in equal quantities T1 
3. The following results were obtained for different kinds of rocks ; 
Calcareous rocks. 
Chalk, as it exists in the general mass of chalk, but well dried . . 
Clunch, from the lower portion of the chalk formation (very moist) 
Oolites from three different beds in the Ancaster quarries . . . 
Statuary marble 
Very hard blue mountain limestone from Derbyshire 
* The temperatures are eipressed in degrees of Faheenheit’s thermometer, 
5 N 2 
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