2S HISrORT of the SOCIETT. 



this ftate, and while the heat communicated was (till under th^t 

 of incandefcence, there appeared in the bottom, or hotteft part of 

 the mafs, an incandefcent fpot, which Increafed in fize. The 

 glafs veflel was now removed from the fire, and carried into a 

 dark room, that the light emitted from it might be the more 

 accurately obferved. There the incandefcence was plainly per- 

 ceived, fpreading from the place where it firft began, and gain- 

 ing ground continually, till the whole became very luminous. 

 The heat, when thus diffiifed through the mafs, begins inftantly 

 to diminifh, and the body quickly cools, as a fimilar mafs of any 

 other fubflance would do. Thefe are the appearances obferved 

 in the experiment ; and are what Dr Hutton proceeds to ex- 

 plain. 



It is evident that the incandefcence, which has juft been de- 

 fcribed, is an operation proceeding from the mafs itfelf, and not 

 from the intenfity of the heat communicated to it, for that heat 

 is not fenfibly incandefcept ; whereas the heat which the mafs 

 acquires, after the veflel is removed from the fii'e, is confider- 

 ably luminous. We have here, therefore, a fpecies of kindling 

 like that of burning bodies ; but, at the fame time, diftindtly 

 difFerent from it. In burning, a phlogiftic fubflance is decom- 

 pofed, by means of the oxygenating principle ; and the matter of 

 light, which was contained in that fubftance, being fet at liberty, 

 is emitted in theibrm of light, and heats thofe bodies by which 

 it happens to be extinguifhed or abforbed. But, in this experi- 

 ment, though the mafs is a phlogiftic fubftance, there is no de- 

 rompofition of the phlogifton, no appearance of inflammation ; 

 fo that its incandefcence proceeds from another caufe than that 

 which operates in burning. On attending to the circumftances, 

 however, we fliall perhaps difcover that the phenomena of this 

 experiment are not anomalous, but follow a rule, exemplified in 

 many ijiftances, though not precifely with the fame appear- 

 ances. 



This 



