Specific Heat and Conducting Power of Building Materials. 319 



inquiries respecting the conducting powers of the substances under 

 examination. 



The plan usually adopted in this kind of research, namely, ob- 

 serving by the aid of thermometers the time occupied by the passage 

 of a certain amount of heat lengthways through the substance of a 

 prism, one end of which was exposed to a high and constant tem- 

 perature, having failed on trial with these bodies, in consequence of 

 their feeble conducting powers, the following method was had re- 

 course to with perfect success : — The various substances examined 

 were cut with the greatest care into cubes of 2*8 inches in the side, 

 and a hole drilled in the centre of one of the faces half way through, 

 large enough to receive the bulb of an exceedingly sensitive thermo- 

 meter, together with a little mercury to improve the contact with the 

 substance of the cube. The temperature of the mass being exactly 

 observed, it was next plunged, all but its upper surface, into a large 

 bath of mercury heated by steam, whose temperature remained con- 

 stant at 211°, and the time of rise of the thermometer for every suc- 

 cessive 10° accurately noted until the maximum was reached, thus 

 affording a comparison of the relative conducting powers, or perhaps 

 more properly, resistance to the passage of heat towards the centre 

 of the mass. 



In the course of these experiments a very extraordinary circum- 

 stance was observed : although the greatest care was taken to equal- 

 ize the temperature of the cubes by suffering them to remain at least 

 twenty-four hours before experimenting in an uniform temperature, 

 yet they never exactly acquired that of the room, or even agreed 

 among themselves in this respect ; an observation which led the au- 

 thor to the suspicion that the generally received doctrine of an equal 

 distribution of sensible heat among bodies in contact and not influ- 

 enced by external sources of disturbance, might not prove strictly 

 true, but that, on the contrary, each of a number of different sub- 

 stances, exposed under similar circumstances to the influence of a 

 medium of uniform temperature, acquires a proper temperature of its 

 own. The same thing was observed with higher degrees of heat ; a 

 mass of slate, for example, plunged beneath the surface of uniformly 

 heated mercury and maintained there long after the thermometer in 

 the slate had reached its maximum, always exhibited a temperature 

 decidedly below that of the surrounding metal*. 



A third series of experiments were made with a view of ascer- 

 taining the relative rates of cooling in air of the various materials 

 examined, from a higher temperature to that of the atmosphere. 

 The arrangement consisted of the cubes before described, covered 

 externally with thin paper for the sake of uniformity of surface, the 

 same delicate thermometer being inserted in the hole in the centre, 

 together with a little mercury for the sake of contact. The cubes 

 were each in turn heated in the steam-chest used for the specific 

 heat experiments, until the included thermometer rose to 200° ; they 

 were then removed, suspended in the air, and the time of fall of tem- 

 perature for every 10 degrees carefully no'.ed. 



[* On this subject a paper by Mr. Parnell was subsequently read, an 

 abstract of which will appear in a future Number. — Edit.] 



