of Porcelain, Gilded and not Gilded. 249 



dom. Besides, it often happens that a very large surface 

 of the warm body is in contact with the cold body ; but, 

 even in this case, there is nothing in the action taking 

 place in the communication of the heat which resembles 

 in any way the sudden explosion which takes place on 

 the restoration of the equilibrium among the particles 

 of an elastic fluid ; on the contrary, the slow and meas- 

 ured progress of this communication, as well as all the 

 other phenomena that it presents, denote rather a grad- 

 ual operation, like that which takes place when the mo- 

 tion of a body is accelerated or retarded. 



The following experiment may serve to explain and 

 confirm this important truth. If a ball of iron, three 

 or four inches in diameter, fastened to a long handle of 

 the same metal, be heated strongly in a forge until it is 

 of a whitish-red heat, and then taken from the fire and 

 plunged suddenly into cold water, the communication 

 of the heat to the water will be so far from being instan- 

 taneous that a considerable time will pass before the ball 

 ceases to be red and luminous at its surface ; and even 

 after the surface of the ball has cooled so far as no 

 longer to give off visible light, the interior will still be 

 incandescent. It is easy to establish this last fact; for 

 if at this point the ball be taken from the water and 

 held in the air for a few moments, the surface of the 

 ball will again become red and luminous. 



I confess frankly that I have never been able to rec- 

 oncile these phenomena with that hypothesis which sup- 

 poses that the increase of temperature of a body is due 

 to the accumulation within it of a very rare and ex- 

 tremely mobile substance, especially when I have con- 

 sidered the great ease with which such a fluid ought to 

 pass through the pores of all known bodies. 



