30 THE RADIATIONS OF IGNITED BODIES. [MBMOIB I. 



being of a beautiful blue ; and even when the barrel had 

 become bright red I could still see the spar, which had 

 decrepitated to a coarse powder, fey its faint blue rays. 

 In these cases, however, it was not incandescence, but 

 phosphorescence that was taking place. I infer, then, 

 that all solids, and probably melted metals, begin to 

 shine at the same thermometric point. 



(When phosphorescent substances are to be examined, 

 they must be first exposed to a high temperature and 

 carefully guarded from access of light until they are 

 placed in the gun-barrel. A diamond which, among oth- 

 er bodies, had been thus tried, would recover its quality 

 of phosphorescing by a very short access of light after it 

 had been cooled, but if that had been carefully avoided, 

 it began to shine at the same time as other specimens 

 with which it was placed in the barrel.) 



The temperature of incandescence seems to be a natu- 

 ral fixed point for the thermometer; and it is very inter- 

 esting to remark how nearly this point coincides w 7 ith 

 1000 of the Fahrenheit scale when Laplace's coefficient 

 for the dilatation of platinum is used. Upon that coef- 

 ficient the point of incandescence is 1006 Fahr. 



In view of these considerations, and recollecting that 

 the number given by Daniel is 980, and that of Wedg- 

 wood 947, I believe that 977 is not very far from the 

 true temperature at which solids begin to shine. It is 

 to be understood, of course, that this is in a very dark 

 room. 



I pass now to the second proposition. The rays emit- 

 ted by the incandescent platinum strip were received on 

 a flint-glass prism, placed so as to give the minimum de- 

 viation, and, after dispersion, viewed in a small telescope. 

 A movement could be given to the telescope, which was 

 read off on a graduated circle. However, instead of 

 bringing the parts of the spectrum tinder measurement 



