MEMOIR I.] THE KADIATIONS OF IGNITED BODIES. 29 



bodies vary in their point of incandescence, I took a clean 

 gun-barrel, and having closed the touch-hole, exposed the 

 following substances in it to the action of a fire : plati- 

 num, chalk, marble, fluor-spar, brass, antimony, gas-carbon, 

 lead ; each specimen was small : the platinum was in the 

 form of a coil of stout wire. 



When one of these bodies was placed in the gun-bar- 

 rel and the temperature raised, it is clear that any differ- 

 ence in their point of incandescence could be detected by 

 the eye. Thus if the ignition of platinum required a 

 higher degree than that of iron, on looking down the 

 barrel the coil of wire should be dark when the barrel 

 itself had begun to shine ; or, if the platinum was incan- 

 descent first, the wire should be seen before the barrel 

 had become visibly hot, and these results might be cor- 

 roborated by observing the inverse phenomena, when the 

 barrel was taken from the fire and suffered to cool. 



In Fig. 2, a t> is the gun-barrel passing through a hole, 

 0, of suitable size in the side 

 of a stove. At the bottom of 

 the barrel, &, the substances to 

 be examined are placed. Their 

 ignition is observed by looking 

 in at the projecting end, a. 



With respect to platinum, 

 brass, antimony, gas - carbon, 

 and lead, they all became in- 

 candescent at the same time as 

 the iron barrel itself. I could 

 not discover the slightest dif- 

 ference among them, either in heating or cooling ; and it 

 is worthy of remark that the lead was, of course, in the 

 liquid condition. But the chalk and marble were vis- 

 ible before the barrel was red-hot, emitting a faint white 

 light; and the fluor-spar still more strikingly so, its light 



