406 Of the Propagation of Heat 



is known to afford so ready a passage to the electric 

 fluid, would also have afforded a ready passage to Heat. 



The common experiments of heating and cooling 

 bodies under the receiver of an air-pump I conceive to 

 be inadequate to determining this question ; not only on 

 account of the impossibility of making a perfect void of 

 air by means of the pump, but also on account of the 

 moist vapour, which, exhaling from the wet leather and 

 the oil used in the machine, expands under the receiver, 

 and fills it with a watery fluid, which, though extremely 

 rare, is yet capable of conducting a great deal of Heat : 

 I had recourse, therefore, to other contrivances. 



I took a thermometer, unfilled, the diameter of whose 

 bulb (which was globular) was just half an inch, Paris 

 measure, and fixed it in the center of a hollow glass ball 

 of the diameter of if Paris inch, in such a manner that, 

 the short neck or opening of the ball being soldered 

 fast to the tube of the thermometer 7} lines above its 

 bulb, the bulb of the thermometer remained fixed in 

 the center of the ball, and consequently was cut off 

 from all communication with the external air. In the 

 bottom of the glass ball was fixed a small hollow tube 

 or point, which projecting outwards was soldered to the 

 end of a common barometer tube about 32 inches in 

 length, and by means of this opening the space between 

 the internal surface of the glass ball and the bulb of the 

 thermometer was filled with hot mercury, which had 

 been previously freed of air and moisture by boiling. 

 The ball, and also the barometrical tube attached to it, 

 being filled with mercury, the tube was carefully in- 

 verted, and its open end placed in a bowl in which there 

 was a quantity of mercury. The instrument now be- 

 came a barometer, and the mercury descending from the 



