VOL. LXXXII.] PHILOSOPHICAL TRANSACTIONS. 135 



about the touching part ; he would not have made use of this expression if the 

 luminous sparks, which issued from them, had not kindled gunpowder and in- 

 flammable air, and thus proved that the part from which they came was raised to 

 a temperature, at least equal to what is usually termed a red heat. If the 

 velocity of the v.'heel be much increased, the touching part of the body applied 

 emits a bright white light, much more vivid than any which powders ever give 

 out on the heater, and probably the temperature of the luminous part is equal to 

 what is usually called a white heat. 



Having thus made incombustible bodies red-hot \vithout the aid of fire, Mr. 

 W. once conceived that all the light which they emit when heated to redness in 

 the fire, proceeded entirely from their great phosphorism ; for he could not sup- 

 pose that they absorbed light from the burning fuel and emitted it again, at the 

 same time, and during a continuance of the same circumstances. It appeared 

 however equally inexplicable, why a stone put into the fire, should continue to 

 shine from its own light, with undiminished lustre, as long as the fire is kept 

 up ; for it has been shown, that if a phosphorescent body remain long on the 

 heater, of any temperature between 400° of Fahrenheit and a red heat, its light 

 diminishes more and more, till at last it is scarcely perceptible ; and then an in 

 crease of heat is necessary to render it more luminous. 



IV. Experiments on Heat. By Major-Generai Sir BenJ. Thompson, Knt 

 F.R.S. Dated Munich, June \7 87. p. 48. 



The first great object which I had in view in this inquiry was to ascertain, if 

 possible, the cause of the warmth of certain bodies ; or the circumstances on 

 which their power of confining heat depends. This, in other words, is no 

 other than to determine the cause of the conducting and non-conducting power 

 of bodies. Having discovered that the Torricellian vacuum is a much worse 

 conductor of heat than common air, and having ascertained the relative con- 

 ducting powers of air, of water, and of mercury, under different circumstances, 

 I proceeded to examine the conducting powers of various solid bodies, and par- 

 ticularly of such substances as are commonly made use of for clothing. 



The method of making these experiments was as follows : a mercurial ther- 

 mometer, whose bulb was about -.^^ of an inch in diameter, and its tube, about 

 10 inches in length, was suspended in the axis of a cylindrical glass tube, about 

 ■I of an inch in diameter, ending with a globe \-^ inch in diameter, in such a 

 manner that the centre of the bulb of the thermometer occupied the centre of 

 the globe ; and the space between the internal surface of the globe and the sur- 

 face of the bulb of the thermometer being filled with the substance whose con- 

 ducting power was to be determined, the instrument was heated in boiling water. 



