118 HEAT OR CALORIC, 



spread upon oiled leather straps, which serve as bands to the 

 wheels.* 



(c.) Jill bodies begin to shine by heat at the same temperature. 

 This fact was first discovered by Sir Isaac Newton, and has been 

 confirmed by others. 



In genera], redness, that is the emission of red rays, commences at 

 about 800 of Fahr. and is fully established in broad day light at 

 1000 in the direct sun's light, perhaps about 1100, or possibly 

 1200. The appearance is of course much influenced by the quan- 

 tity of the surrounding light. A body might be luminous in the dark, 

 that would not be at all so in the light. 



There are many cases of phosphorescence or emission of light 

 which are not attended by any considerable increase of heat ; these 

 have been already mentioned under the head of light. 



(d.) A white heat is only a greater degree of ignition. White 

 light, that is, light containing a due proportion of all the colored rays, 

 is emitted when the accumulation of heat is the greatest ; a welding 

 heat of iron is a white heat. The artists have many terms to de- 

 note the various degrees of heat connected with their processes ; 

 thus, they speak of a cherry red, a worm red, &c. and of a white 

 heat, a blue white, a red white, &tc. and there are many degrees of 

 heat between, commencing with the feeblest redness visible only in 

 the dark, and ending with a full white light, distinctly visible even in 

 the blaze of the meridian sun.f 



(e.) Ignition affords one of the strongest arguments for the iden- 

 tity of light and heat. If they are different substances or powers, 

 then the heat when accumulated to a certain degree, expels the light, 

 previously lodged in the body ; or, it may be said, that as most cases 

 of ignition are produced by burning bodies, the light from the fire 

 enters the body along with the heat, and thus obtains a transit ; or, if 

 heat and light are merely modifications of each other, then it may 

 be supposed that at a certain temperature heat becomes light, or pos- 

 sibly a certain accumulation or intensity of radiant heat affects the 

 optic nerves so as to produce the sensation of vision. J 



* This is beautifully seen at the gun manufactory, at Whitneyville, near New 

 Haven ; the sparks fly off in innumerable tangents, and the hand, unless brought 

 very near, may be held in the fiery stream without inconvenience ; this is doubtless 

 owing to the strong current of air which the revolution of the wheels produce. It 

 is curious that while coarse emery is used, gunpowder is inflamed by the sparks at 

 any distance to which they extend ; but, when very fine emery is used, coarse gun- 

 powder is not kindled, but if finely pulverized, it then flashes with the minutest 

 sparks. (Communicated by Mr. Eli Blake of Whitneyville.) 



t Although it is called a white heat, there are more red rays than are contained 

 in the sun beams. 



The very mild heat which causes the emission of light from some bodies, e. g. 

 fluor spar, countenances the opinion that light is lodged in them ; and light may be 

 imparted to some bodies to such a degree, that they become partially transparent 

 without producing, upon them, the effects of ignition ; thus, eggs, the human fingers, 

 and other bodies are illuminated, through and through, by an electrical discharge. 



