S92 Methid of nie:i/iiring the cmdu^hig Powtrs of Fluids. 



joj qiiaiiiy, by aclual motiom, from the heated to the cold bodies with which they may fuc» 

 cefUvcly come into coiuacl ; and that every means of obllruding or retarding thofe motions 

 would render the propagation of heat more flow and dillicult. lie had before found that 

 this was a^flually the cafe with air, and on the prcfent occafion he proceeded to give the f jb- 

 je£l a thorough and careful inveftigation. 



The apparatus made ufe of for the firft courfe of experiments confided of a large ther- 

 mometer, which lie calh tiie padhgc thermometer, confiding of a cylindrical vcflel with he- 

 mifpherical ends, forming the bulb, and a glafs tube fitted into a neck by means of a good 

 cork. Its dinicnfions were as follow : — Diameter of the bulb 1.84. inches — Length 4.99— 

 Capacity or contents 13.2099 cubic inches — External fuperficies 28.834 fuperficial inches 

 — riiic!:nefs of the fliect copper 0.03 inch — Weight when empty 1846 grains, and it con- 

 tains 3344 grains of water at 55" of temperature. The glafs tube is 2). inches long, and 

 ■j^jths of an inch in diameter, and the cylindrical neck of copper into which it is fitted by 

 means of cork is one inch long, and -rVedths of an inch in diameter. 



This thermometer, being filled with linfecdoil, and its fcale graduated, was fixed in the 

 axis of a hollow cylinder of thin flieet copper, \i\ inches long, and 2.3435 inches in dia- 

 meter internally. This cylinder, which is open at one end, is clofed at the otlier with art 

 hcmifpherical bottom with its convex fide outwards. The bulb of the thermometer was 

 confined in the axis of this cafe, by three fmall wooden pins, inferted in fockets within 

 the large brafs tube, and the upper end was properly fecurcd by caufing the glafs tube to pafs 

 through a cork flopper adapted to the fame metallic cylinder. The bottom of the bulb 

 relied on a wooden pin fixed in a focket in the middle of the hcmifpherical bottom of the 

 cafe. All the pins terminated in blunt wooden points, to reduce the contafts as much as 

 poDTible. 



The fpace between the thermometer when in its place, and the internal furface of the 

 furrounding cylinder, was dcfigned to contain the fubftance through which the heat was made 

 to pafs into or out of the thermometer; the temperature of this lad niafs being flievin by 

 the graduations on the glafs tube. The quantity of water required to fill the fpace, and 

 cover tlie bulb of the thermometer about one quarter of an inch, was found to weigh 2468 

 grains. 



When tlie bulb of the thermometer was furrounded in its place by water cr any other 

 liquid or mixture intended to be tried, a cylinder of cork, rather lefs in diameter than the 

 internal cavity, was (lipped down upon the tube, not quite fo low as the water or mixture." 

 Above this was placed a quantity of eider-down, fufiicient to fill the remaining cavity, ex- 

 cept what was occupied by the cork ftopper, lall of all to be inferted. The thermometer 

 W.IS divided according to Fahrenheit's fcale, and the whole fcale, from the freezing point to 

 the boiling-water point, was above the flopper. 



The operations with this apparatus were performed by placing tlie prepared inflrumcnt 

 in melting ice till the thermometer fell to 32°. It was then immediately plunged into a large 

 vcflel of boiling water, and the condu£ling power of the fubdance under examination was 

 eftimaled by the time the heat employed in patTmg through it into the thermometer; the 

 time being carefully noted when the liquid in the thermometer arrived at the 40th degree 

 of its fcale ; and alfo when it came to every 20th degree above it. 



In the rcvcrfe operation, the indrument was kept in boiling water till its temperature 



appeared 



