BURGESS AND FOOTE : EMISSIVITY OF METALS 279 



liquid with the petrographic microscope. Of these methods, 

 method 5 which requires solely a plane-parallel glass plate of 

 high refractive index, with one edge beveled at an angle of 60°, 

 is the simplest and most convenient. With it the refractive in- 

 dex of the liquid can be easily measured to the third decimal 

 place, an accuracy which is sufficient for all determinative pur- 

 poses. The other methods are interesting and of about the same 

 order of accuracy but they are less simple and therefore less use- 

 ful. The methods of Clerici, Viola, Piltschikow, Smith and 

 Clay are presented briefly but they are also without exception 

 less simple and less accurate than method 5, which is accordingly 

 the method recommended, especially as the cost of the glass plate 

 required is shght and the amount of liquid necessary for a meas- 

 urement is a very small drop. It is of the greatest importance 

 to the science of petrology that the materials, of which it treats, 

 be studied in a quantitative way; the refractive indices of the 

 rock making minerals, many of which are isomorphous mixtures, 

 are fundamental optical constants which should be measured 

 and included in every detailed description of a rock. 



PHYSICS. — The emissivity of metals and oxides. I. Nickel 

 oxide (NiO) in the range 600 to 1300° C. G. K. Burgess 

 and P. D. Foote. (To appear in full in the Bulletin of the 

 Bureau of Standards.) 



The object of the present investigation has been the deter- 

 mination of the monochromatic (E^) and total emissivity 

 (E) of nickel oxide (NiO) in the range 600 to 1300°G. This 

 oxide forms a tough, smooth layer on the surface of nickel 

 when subjected to high temperatures in air. Two methods were 

 employed for the determination of the emissivity for red light: 

 (a) that of microscopic melts, and (b) a direct comparison by a 

 spectrophotometer of the intensity of light emitted by the 

 glowing NiO and by a black body at the same temperature. 



In the method of microscopic melts, minute specimens of 

 XaCl, Na2S04, and Au were placed on the oxide which was elec- 

 trically heated until these substances melted, as observed with 

 a microscope, and the "apparent" temperatures at the instant 



