SECT. II.] PRELIMINARY DEFINITIONS. 31 



it ; whence, if it is formed of ice, it becomes liquid ; but if this 

 plate of ice is exposed to the rays of a torch it allows a sensible 

 amount of heat to pass through with the light. 



36. We have taken as the measure of the external conduci- 

 bility of a solid body a coefficient h, which denotes the quantity of 

 heat which would pass, in a definite time (a minute), from the 

 surface of this body, into atmospheric air, supposing that the sur 

 face had a definite extent (a square metre), that the constant 

 temperature of the body was 1, and that of the air 0, and that 

 the heated surface was exposed to a current of air of a given in 

 variable velocity. This value of h is determined by observation. 

 The quantity of heat expressed by the coefficient is composed of 

 two distinct parts which cannot be measured except by very exact 

 experiments. One is the heat communicated by way of contact to 

 the surrounding air : the other, much less than the first, is the 

 radiant heat emitted. We must assume, in our first investigations, 

 that the quantity of heat lost does not change when the tempera 

 tures of the body and of the medium are augmented by the same 

 sufficiently small quantity. 



37. Solid substances differ again, as we have already remarked, 

 by their property of being more or less permeable to heat ; this 

 quality is their conducibility proper: we shall give its definition and 

 exact measure, after having treated of the uniform and linear pro 

 pagation of heat. Liquid substances possess also the property of 

 transmitting heat from molecule to molecule, and the numerical 

 value of their conducibility varies according to the nature of the 

 substances : but this effect is observed with difficulty in liquids, 

 since their molecules change places on change of temperature. The 

 propagation q heat in them depends chiefly on this continual dis 

 placement, in all cases where the lower parts of the mass are most 

 exposed to the action of the source of heat. If, on the contrary, 

 the source of heat be applied to that part of the mass which is 

 highest, as was the case in several of our experiments, the transfer 

 of heat, which is very slow, does not produce any displacement, 

 at least when the increase of temperature does not diminish the 

 volume, as is indeed noticed in singular cases bordering on changes 

 of state. 



