116 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1786. 



air of different degrees of density. For this experiment I prepared the thermo- 

 meter N° 4, by stopping up one of the small glass tubes passing through the stop- 

 ple, and opening a passage into the cylinder, and by fitting a valve to the exter- 

 nal overture of the other. The instrument, thus prepared, being put under the 

 receiver of an air-pump, the air passed freely out of the globe and cylinder on 

 working the machine, but the valve above described prevented its return on let- 

 ting air into the receiver. The gage of the air-pump showed the degree of rarity 

 of the air under the receiver, and consequently of that filling the globe and cylin- 

 der, and immediately surrounding the thermometer. 



With this instrument, the weather being clear and fine, the mercury in the 

 barometer standing at 27 inches 9 lines, the thermometer at 15°, and the hygro- 

 meter at 47°, I made the following experiments. 



(Exp. N° 22.) 

 Thermometer N° 4. 

 Surrounded by air rarefied by 

 pumping till the barometer-gage 

 stood at 1 inch 2 lines. 



Taken out of freezing water, 

 and plunged into boiling water. 



Time elapsed. 



36' 

 49 

 1 

 1 

 24 

 31 



not observed. 



Heat acquired. 

 10° 

 20 

 30 

 40 

 50 

 60 

 70 

 80 



7 51= total time of heat- 

 ing from 0° to 70°. 



The result of these experiments, I confess, surprised me not a little. I hope 

 that further experiments may lead to the discovery of the cause why there is so 

 little difference in the conducting powers of air of such very different degrees of 

 rarity, while there is so great a difference in the conducting powers of air, and 

 of the Torricellian vacuum. 



I shall conclude this letter with a short account of some experiments I have 

 made to determine the conducting powers of water and of mercury ; and with a 

 table, showing at one view the conducting powers of all the different mediums 

 which I have examined. Having filled the glass globe inclosing the bulb of the 

 thermometer N° 4, first with water, and then with mercury, I made the fol- 

 lowing experiments, to ascertain the conducting powers of those two fluids. 



