28 Messrs. W. E. Wilson and P. L. Gray. On the 



anything beyond moderately low temperatures ; we therefore 

 assumed in our former work that the form of the law was the same 

 for a blackened as for a bright surface, there being good grounds, 

 both theoretical and experimental, for such a belief. Further inves- 

 tigations, however, indicate that this assumption is not correct, as 

 will be seen from the experiments detailed below. 



A series of experiments on the radiation from bare platinum was 

 made first, exactly in the same way as those described in oar work 

 of last year, that is to say, the radiation from the platinum at 

 different temperatures was allowed to fall on a radio-micrometer of 

 the ordinary form, the sensibility of which was reduced sufficiently 

 to give a readable deflection at the highest temperature used, the 

 deflection as given by the scale-readings being then taken as propor- 

 tional to radiation. This proportionality has been shown before to be 

 strictly true for deflections up to and greater than those obtained in 

 these experiments. 



The platinum- strip was next blackened on one side with black 

 oxide of copper, which was ground very fine, mixed up with methy- 

 lated spirit, and laid on with a camel's-hair brush ; this, when the 

 liquid had dried off, gave a very good, even, dead-black surface, the 

 emissive power of which may be taken as approximately equal to that 

 of an ideal black surface. 



Lampblack, of course, is useless for these experiments, since it 

 burns off at something under 500 C. ; it could only be used if the 

 radiator could be placed in a vacuum, or in an atmosphere having no 

 action on the carbon, for which purpose we are having apparatus 

 specially constructed. 



At about 900 C. the black oxide of copper begins to suffer a 

 change ; its surface becomes somewhat shiny, and an alloy is formed 

 with the platinum ; this puts a limit to the temperature at which the 

 radiation may be taken as that of a "black" surface. Our first 

 strip was spoiled in discovering this limiting temperature ; the second 

 strip (after calibration, &c., and radiation experiments with the metal 

 bare) was covered on both sides and examined during the progress of 

 the experiments, which was stopped as soon as the black surface 

 showed any signs of change of physical condition ; these were not 

 only apparent to the eye, but were also immediately indicated by a 

 variability of temperature, due to the alteration of emissive power, as 

 the reduction of the oxide crept over the surface of the strip. 



Platinum-black would have no advantages in this connection over 

 the copper-oxide, as it reverts to the metallic condition at very nearly 

 the same temperature as that at which the oxide changes in the way 

 mentioned above. 



The two series of experiments gave the figures in the following 

 tables : 



