^2 HisroRT of the society: 



of metals. When the mixture of the metaJ and fulphur is ex- 

 pofed to heat, and the fulphur melted, it is not immediately 

 combined with the metal j for it requires a greater degree of heat, 

 and one which is perhaps nearly that of incipient incandef- 

 cence, to produce the compound flibftance of the fulphurated 

 metal. The moment, however, that this combination is form- 

 ed, that part of the mafs, in which it began, lofes its fluidity,, 

 and is made to concrete, and at ijhe fame time becomes ftrong- 

 ly incandefcent. In this flate, if tlie glafs vefTel be removed, 

 from the fire, the procefs of the combination of the fulphur 

 with the metal will be carried on, as has been defcribed above.. 

 For the firft incandefcent part is that which had been moft im- 

 mediately expofed to the heat of the burning coals, and had, by 

 that means, acquired the temperature neccffary for the combi- 

 nation of the two fubflances ; but at this time the part imme- 

 diately contiguous to the concreted portion of the mafs is in 

 the next degree of heat ; confequently, upon the emerging of 

 the latent heat of the firft portion, this fecond portion, having 

 its heat increafed, is made to combine, and, by its inftant con- 

 folidation, produces incandefcence. This incandefcence of the 

 fecond portion produces a like effecft upon the third ; and thus- 

 the heat, combination, concretion, and incandefcence, fpread 

 quickly through the whole mafs, without any further affiftance 



from external fire. 



In all this there is fo great a refemblance to the phenomena 

 of burning, that ibme attention is necefiTary to enable an obfer- 

 ver to diftinguifh the one procefs from the other. 



When a mafs of charcoal, properly prepared for combuftion, 

 is kindled in one part by the heat of incandefcence, the oxyge- 

 nation begins, attended with the decompofition of phlogifton, 

 and the emiffion of the fixed light. The neighbouring parts, 

 being then heated, by the light emitted from the firft kindled 

 part, are alfo kindled themfelves, and ferve to augment both 



the 



