Luminous Radiation at a Red Heat. 137 



To be more definite, we might, I think, assume that repre- 

 senting by 100 the normal emissive power of oxide of iron and 

 of the bodies we have classed with it, that of platinum would 

 be about 32, that of gold 10, while that of oxide of zinc would 

 not exceed 5, if indeed it amounted to this number. I may 

 finally add that a coating obtained by mixing together chalk, 

 a little borax, and water, appeared in one experiment to approach 

 oxide of zinc in the feebleness of its emissive power. 



In the researches the results of which we published in 1854, 

 we heated to redness the coatings whose emissive power we 

 desired to study, by applying them on a small plate of gold or 

 of platinum, which could be traversed by an electric current. 

 In my new experiments, without abandoning this mode of heat- 

 ing, I usually preferred to apply the oxides submitted to my ob- 

 servations on plates of gold or platinum which were heated by 

 eolipyles. These plates, which were tolerably thick, were not 

 appreciably altered in shape during the experiments. 



When, without attempting to make any measurements, it is 

 simply desired to show the difference of emissive power for light, 

 a plate of this kind is taken of about 25 or 30 square centims., 

 and on one of its faces two lines are traced at right angles passing 

 through its middle, and thus dividing it into four contiguous 

 portions; on one of these the metallic lustre is left,, the second 

 is covered with oxide of zinc, the third with oxide of iron, the 

 fourth with oxide of copper. When the coatings are dry, the 

 plate is placed in a dark chamber and heated with a flame of 

 alcohol or of gas, which strikes against that face which has no 

 coating. As soon as the temperature had reached a red heat, alL 

 the effects which we have indicated were observed. 



If photometric observations are to be made, it is important to 

 take precautions for preventing rays emitted from the source of 

 heat from being mixed with those which come from the surfaces 

 investigated. In this case the plate may be fixed at the end of 

 an earthen tube whose internal surface has been blackened or 

 covered with oxide of iron. The external face of the plate is 

 heated, and the other is observed by placing the photometer 

 near the open end of the tube. 



The extreme smallness of the luminous emissive power of oxide 

 of zinc has surprised me ; but although I have greatly varied 

 my experiments on this point, they have all led to the same 

 result. I shall not detail them ; I will merely mention one 

 which appears to prove that the small emissive power of this 

 oxide is not at all due to the fact that, from want of conducti- 

 bility, it only attains at its external surface a temperature 

 materially lower than that of other coatings applied on the same 

 body. 



