in various Substances. 435 



In determining the relative conducting powers of 

 these mediums, I have compared the times of the 

 heating of the thermometer from 0° to 70° instead of 

 taking the whole times from 0° to 80°, and this I have 

 done on account of the small variation in the Heat of 

 the boiling water arising from the variation of the weight 

 of the atmosphere, and also on account of the very slow 

 motion of the mercury between the 70th and the 80th 

 degrees, and the difficulty of determining the precise 

 moment when the mercury arrives at the 80th degree. 



Taking now the conducting power of mercury = 1000, 

 the conducting powers of the other mediums, as deter- 

 mined by these experiments, will be as follows, viz. : — 



Mercury ...... looo 



Moist air . . . . . . 330 



Water ...... 313 



Common air, density = i . . Soy^'g- 



Rarefied air, density = |- . . . 8oj2_3__. 



Rarefied air, density = J^i • • 78 



The Torricellian vacuum ... 55 



And in these proportions are the quantities of Heat 

 which these different mediums are capable of transmit- 

 ting in any given time; and consequently these numbers 

 express the relative sensible temperatures of the mediums, 

 as well as their conducting powers. How far these de- 

 cisions will hold good under a variation of circum- 

 stances, experiment only can determine. This is cer- 

 tainly a subject of investigation not less curious in 

 itself than it is interesting to mankind ; and I wish that 

 what I have done may induce others to turn their at- 

 tention to this long neglected field of experimental 

 inquiry. For my own part I am determined not to 

 quit it. 



