OS THE SOURCES AND EFEECTS OF HEAT. 635 



mined. We shall find that this communication happens in one of both of two 

 Avays, by contact, or by radiation; and that it may also differ both with re- 

 spect to the quantity of heat concerned, and to the time occupied by the pro- 

 cess. Whatever heat may be, we may safely conclude that in substances of 

 the same kind, at the sailie temperature or apparent degree of warmth or 

 coldness, its quantity must be proportional to tlae mass or weight; for instance, 

 that a quart of the water of a given cistern contains twice as much heat as a 

 pint; and where this is true of the different parts of any substance, they must 

 remain in equilibrium with respect to heat. But if two equal portions of the 

 same substance, containing different quantities of heat, be in contact, they will 

 affect each other in such a manner as to have their temperatures equalised, 

 and the more rapidly as the contact is more perfect. Thus, if two portions 

 of a fluid at different temperatures be mixed together, they will acquire imme- 

 diately an intermediate temperature; and when two solids are in contact, the 

 quantity of heat, communicated by the hotter to the colder in a given time, is 

 nearly proportional to the difference of the temperatures. Hence it would 

 follow, that they could never become precisely of the same temperature in 

 any finite time; but in fact the difference of temperature is rendered, in a mo- 

 derate time, too small to be perceptible. The nature of the substances con- 

 cerned has also a material effect on the velocity with which heat is commu- 

 nicated through their internal parts; metallic bodies in general conduct it 

 the most readily, earthy and vitreous bodies, the least; but the various metals 

 possess this power in different degrees; silver and copper conduct heat more 

 rapidly than iron, and platina transmits it but very slowly. Professor 

 Pictet supposes that heat ascends within solid bodies more readily than it de- 

 scends; but the effect of the air remaining in the imperfect vacuum of the 

 air pump may be sufficient to explain his experiments; the difference of terrt- 

 perature producing an ascending current in the neighbourhood of the heated 

 body, by means of which the cold air continually approaches its lower parts, 

 and carries the heat upwards: and it has been found that the rarefaction of air 

 does not by any means diminish its power of conducting heat, in proportion 

 to the diminution of its density. 



Count Rumford's experiments have shown that all fluids are very imperfect 

 conductors of heat by immediate contact, although it is scarcely credible that 

 they can be absolutely nonconductors; but heat is usually communicated 



