^8 HIS TORT of the SOCIETK 



this ftate, and while the heat communicated was ftill under thart: 

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

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

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

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

 accurately obferved. There the incandefcence was plainly per- 

 jceived, fpreading from the place where it firll began, and gain- 

 ing ground continually, till the whole became very luminous. 

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

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

 other fubftance 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 incandefceyit ; whereas the heat which the mafs 

 acquires, after the velTel is removed from the fire, is confider- 

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

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

 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 fubflance, being fet at liberty, 

 is emitted in the rform of hght, and heats thofe bodies by which 

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

 ment, though the mafs is a phlogiftic fubflance, 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 circum^flances, 

 however, we fhall perhaps difcover that the phenomena of this 

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

 many inflances, though not precifely with the fame appear- 

 ances. 



This 



