420 Messrs. Nichols and Snow on the Influence of 



The changes in passing from 25° to 469° C. were found to 

 be slight. Readings were made in three portions of the 

 spectrum only, but in each of these the reflecting-power of 

 the hot lamp-black is less than that of the cold. See 

 Table XII. 



Table XII. — Reflecting-power of Lamp-Black in terms of 

 that of the ideal White. Temperatures, 25° and 469° C. 



Wave-lengths. 



Intensities at 



25° C. 



Intensities at 

 469° C. 



■6685 

 •5570 

 •4920 



0-0055 

 0-0072 

 0-0073 



0-0053 

 00061 

 0-0069 



Zinc oxide was one of the pigments from which we ex- 

 pected to obtain very definite and well-marked results. The 

 change of this substance from white to lemon-yellow, under 

 the blowpipe, is one of the most familiar of colour-reactions. 

 We anticipated no difficulty in fixing the precise nature of 

 the changes which take place with rise of temperature. 

 Films obtained by smoking the platinum foil over the flame 

 of burning zinc were studied by the methods which already 

 have been described in this paper. Our measurements yielded 

 most erratic and, at first sight, inconsistent results. It soon 

 became evident that in the study of the selective reflexion 

 from zinc oxide at different temperatures we had to deal with 

 a set of very complex phenomena ; and it was indeed not 

 until we had completed an investigation of the radiation 

 of zinc oxide at high temperatures, a piece of work the results 

 of which are to form the subject of another paper, that we 

 were in position to understand the behaviour of that substance 

 as regards reflecting-power. Our study of the radiation of 

 the oxide at temperatures above the red heat showed the light 

 emitted to be strongly selective, in the sense of differing 

 greatly in distribution from that emanating from incandescent 

 carbon or platinum at the same temperatures. It was found 

 also that the character and intensity of the radiation were 

 functions of time as well as of temperature. Every sharp 

 fluctuation in radiating-power had its counterpart in colour- 

 changes of the pigment, as viewed by reflected light, and 

 the difficulty which we had experienced in measuring the 

 pigment arose from the fact that we had to deal with a 



