VOL. LXXXII.] PHILOSOPHICAL TRANSACTIONS. 21Q 



Exper. 12. A bit of iron wire becomes visibly red-hot when immersed in 

 melted glass. Air therefore is not necessary to the shining of ignited bodies. 



Exper. 13. A piece of red-hot metal continues to shine for some time after 

 its removal from the fire ; which proves, that constant accessions of light or heat 

 are not necessary to the shining of ignited bodies. If the piece be strongly 

 blown on, it instantly ceases to shine ; for the cold air, continually applied, 

 unites with the light as fast as it leaves the body, and which otherwise would 

 have passed to the eye. 



I shall now close this paper with two or three miscellaneous observations. 



Red-hot bodies, though ignited by white light, give out only the red rays. 

 Perhaps the other more refrangible rays, from their greater attraction to matter, 

 may be circulating as heat, while the red ones, having a less attraction, yield 

 sooner to that force which propels the light of red-hot bodies. If the intensity 

 of the incident white light be much increased, so as to raise the body to a white 

 heat, the more refrangible rays then come out with the others, constituting to- 

 gether a white light. 



The flash of a grain of gunpowder is a pure white light: but if the explosion 

 be made within a thin, unglazed, earthen-ware tube, close at both ends, all the 

 light that pervades the sides of the tube is red; the other rays must therefore 

 remain united with the matter of the tube, while the less attractive red ones are 

 transmitted. Thus also, on looking at the sun through the thin bottom of an 

 earthen-ware tea-cup, only the red rays are transmitted, so that the others must 

 be retained by the matter of the cup. It would perhaps be worth trying, whe- 

 ther a body can be made red-hot by concentrated rays of other colours. 



The light produced from bodies by attrition consists of a double light; that 

 which their powder would give out on the heater under redness; and that which 

 particles in their surfaces give out by being made red-hot. The sudden heating 

 of a body to redness, by a single rub or blow, is a remarkable phenomenon, and 

 deserves to be investigated. One effect produced on a body by attrition, is a com- 

 pression or condensation of the parts in its surface; and it appears from general 

 observation, that a condensation of the parts occasions a diminution of its capa- 

 city for heat. Iron may be made red-hot by repeated blows of a hammer; and 

 I have found, that if red-hot iron be forcibly struck by a heavy hammer, with a 

 sharp edge to concentrate the action, the part so struck emits a white light for a 

 sensible time, and is probably raised to a white heat: also, that my father's ther- 

 mometer clay has its capacity for heat diminished 4-, by being burnt to 120° of 

 his scale, and thus reduced to about -J- of its bulk; and as it loses in weight little 

 more than 2 gr. on a pound, the diminution of capacity can only be attributed to 

 its condensation. Many other analogous instances might be adduced if necessary ; 

 but these will perhaps be deemed sufficient to render it probable, that the sudden 



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