concerning Heat. 133 



the time was noted by means of the thermometer, 

 during which the hot water in the small vessel became 

 cold. 



I was careful to plunge the instrument in this frigo- 

 rific mixture, so that the large vessel was completely 

 submerged, except the upper extremity of its neck ; and 

 I added, from time to time, a sufficient quantity of 

 pounded ice to keep the frigorific mixture constantly 

 and throughout at the temperature of melting ice. 



The following were the results afforded by two simi- 

 lar instruments, employed at the same time : 



These two instruments, which I shall distinguish 

 respectively by the letters A and B, are of the same 

 form and dimensions; there is no difference between 

 them but in the state of their surfaces. In the instru- 

 ment A the exterior surface of the small vessel and the 

 interior surface of the great vessel which encloses it 

 are bright and polished; but in the instrument B 

 the exterior surface of the small vessel and the inte- 

 rior surface of the large vessel are black, having been 

 blackened over the flame of a candle, before the bottom 

 of the great vessel was soldered in its place. 



Having filled the interior vessel of each of these in- 

 struments with boiling water till the water rose to the 

 height of half an inch in the neck, I placed a ther- 

 mometer in each ; and then plunging both instruments 

 at the same time into a tub filled with cold water, mixed 

 with pounded ice, I observed the course of their refrig- 

 eration during several hours. 



Each of the instruments was completely submerged 

 in the frigorific mixture, excepting about one inch of the 

 superior extremity of the neck of the exterior vessel, 

 and I was careful to add new quantities of pounded ice, 



