on the Subject of Heat. 197 



kind without discovering the least difference from those 

 mentioned above. It would take too much time and 

 space to describe them all here. They are to be found, 

 however, in my memoir in the Philosophical Transac- 

 tions and in my eighth Essay. 



I had subsequently several instruments of the same 

 sort made, in order to repeat and vary my experiments. 

 Sometimes I observed the time which they took in cool- 

 ing, sometimes that necessary for the heat to penetrate 

 them. Sometimes I performed the experiment in the 

 open air, sometimes in water. All these experiments 

 gave the same result, namely, that the thermometer 

 bulb in a vacuum became warm or cold as the case 

 might be, the only difference being that it always took 

 nearly twice as long to effect this change of temperature 

 as was required when the bulb was -surrounded by air. 



The passage of heat through a vacuum was a fact of 

 such importance in the investigation of the nature of 

 heat, that I wished to confirm it by experiments which 

 would not allow a shadow of doubt. 



That part of the thermometer tube which was in- 

 serted in the glass globe was in contact with this globe. 

 Hence the thought might suggest itself that a part of 

 the heat received or given out by the thermometer bulb, 

 which was surrounded by the vacuum, was. communi- 

 cated by means of the tube of the thermometer, since 

 a portion of this tube was surrounded by air or water in 

 which the heating or cooling was effected. In order to 

 be fully satisfied as far as this circumstance was con- 

 cerned, it occurred to me to repeat the experiment with 

 a thermometer suspended by a very fine silken thread 

 in the middle of a glass body of such size that the 

 thermometer with its tube was entirely contained in it. 



