136 PHILOSOPHICAL TRANSACTIONS. [aNN i7Q2. 



and afterwards being plunged into a freezing mixture of pounded ice and water, 

 the times of cooling were observed, and noted down. 



As this instrument is calculated merely for measuring the passage of heat in 

 the substance whose conducting power is examined, I shall give it the name of 

 passage-thermometer ; and I shall apply the same appellation to all other instru- 

 ments constructed on the same principles, and for the same use, which I may in 

 future have occasion to mention. In most of my former experiments, in order 

 to ascertain the conducting power of any body, the body being introduced into 

 the globe of the passage-thermometer, the instrument was cooled to the tem- 

 perature of freezing water, after which, being taken out of the ice water, it 

 was plunged suddenly into boiling water, and the times of heating from 10 to 

 10 degrees were observed and noted; and I said that these times were as the 

 conducting power of the body inversely ; but in the experiments of which I am 

 now about to give an account, I have in general reversed the operation ; that is 

 to say, instead of observing the times of heating, I have first heated the body in 

 boiling water, and then plunging it into a mixture of pounded ice and ice-cold 

 water, I have noted the times taken up in cooling ; as a method both easier and 

 more accurate. In heating the thermometer, I did not in general bring it to 

 the temperature of the boiling water, as this temperature is variable; but 

 when the mercury had attained the 75° of its scale, I immediately took it out of 

 the boiling water, and plunged it into the ice and water ; or, which I take to be 

 still more accurate, suffering the mercury to rise a degree or 2 above 75°, and 

 then taking it out of the boiling water, I held it over the vessel containing the 

 pounded ice and water, ready to plunge it into that mixture the moment the 

 mercury, descending, passes the 75"- 



Having a watch at my ear which beat half seconds, which I counted, I noted 

 the time of the passage of the mercury over the divisions of the thermometer, 

 marking 70° and every 10th degree from it, descending, to 10° of the scale. I 

 continued the cooling to 0°, or the temperature of the ice and water, in very 

 few instances, as this took up much time, and was attended with no particular 

 advantage, the determination of the times taken up in cooling 6o° of Reau- 

 mur's scale, that is to say, from 70° to 10°, being quite sufficient to ascertain 

 the conducting power of any body whatever. 



My first attempt was to discover the relative conducting powers of such sub- 

 stances as are commonly made use of for clothing ; accordingly, having pro- 

 cured a quantity of raw silk, as spun by the worm, sheep's wool, cotton wool, 

 linen in the form of the finest lint, being the scrapings of very fine Irish linen, 

 the finest part of the fur of the beaver, separated from the skin, and from the 

 long hair, the finest part of the fur of a white Russian hare, and Eider down ; 



