312 Royal Societrj : — 



The apparatus made use of was sufficiently simple. The heat 

 ■was derived from a stove, the fire within which could be elevated, 

 depressed, or eutirel}- withdrawn at pleasure. A very shallow pan 

 of mercury was placed over the stove, the fire being so regulated as 

 to preserve the merciuy at any constant required temperature. A 

 cylindrical block of any substance, the conductive power of which 

 was to be determined, was so placed as to rest with its base just in 

 contact with this mercury, from which it derived its temperature (^,). 

 Its upper end was also covered with sufficient mercury just to cover 

 the small bulb of a thermometer. The temperature of this latter 

 mercury gave t„. Careful arrangements were made for observing 

 these temperatures, as well as that of the air into which the heat 

 radiated from the upper mercury. Precautions were also taken to 

 prevent the lateral transference of heat through the sides of the 

 block, and any influence of radiation from the heated stove which 

 might affect the results of the experiments. When the temperature 

 {t.^ of the upper mercury became stationary, the experiment was 

 completed, and the substitution of this stationary value of f^, together 

 with the values of #, and r in the above formula, gave the numerical 

 results required. 



2. The following were some of the results obtained for conductive 



A' 

 powers as measured by the ratio - : — 



Chalk -056 



Clay -07 



Sand -15 



Sand and clay .... '11. 



These substances were all in the state of verij dnj powder. In the 

 last case the sand and clay were in equal quantities. 



Substances in the state of rock-masses, 



(1) Calcareous roclcs. 



Chalk (same block from a dry state to a state 



of saturation with water) from ' 1 7 to '30 



Oolites from Ancaster (dry to saturated) .... -30 to "40 

 Hard compact limestones '50 to 'bo 



(2) Argillaceous substances. 



Clay, very dry to very moist '23 to '37 



(3) Siliceous rocks. 



New red sandstone (same block dry to satu- 

 rated) " -25 to -(30 



Freestone -33 to -45 



Hard compact sandstones (j\Iillstone-grit). . . . '51 to '76 



(4) Hard, compact, old sedimentary rocks -50 to -61 



(5) Igneous rocks -53 to TOO 



Effect of Pressure. 



3. This effect was not appreciable for a pressure of 7500 lbs. per 

 square inch iu such substances as bees' -wax and spermaceti. Nor 



