MAGNESIUM OXIDE. Ill 



1.5 to 2 ll is worthy of notice. In one case where the ratio of emissivity 

 at 4.3 {x for a given increase in energy is 50, it is almost 200 at 2 /*. In 

 fig. 77 the curves are all for practically the same sensibility, 71, of the 

 instrument. In fig. 78 the sensibility of the instrument was 60. These 

 units are arbitrary, being the galvanometer deflections obtained by un- 

 balancing the bolometer by a constant amount. For this substance the 

 radiating rod was short and the voltage too low to be measured with the 

 electro-static voltmeter. The ordinary voltmeter used affected the tem- 

 perature of the radiator slightly, so that the energy consumption recorded 

 is not so accurate as in the other observations. 



EMISSION SPECTRA OF SOLIDS ON NERNST "HEATER TUBE." 



In examining the radiation from solids placed upon a heater it is pos- 

 sible to have some of the radiation from the latter transmitted through 

 the former. If the substance to be examined is in the form of a fine powder 

 and the layer is from 0.5 to 1.5 mm. thick there is but little chance for 

 the radiation to be transmitted from the heater. This fact enables one to 

 study the selective emission of solids which can not be formed into solid 

 rods, and is applied in studying the following list of substance. 



Zirconium Oxide (Zr0 2 ). 



(Layer of oxide 0.8 to 1.5 mm. thick, on Nernst heater tube from which the clay 



covering had been removed. Fig. 79, curves c, b, and c.) 



The zirconium oxide was heated to a dull red, which was somewhat 



lower than for the rod heated electrically (curve a, fig. 77). Curves a and 



c give the emission of different samples of the commercial product, the 



electrical properties of the latter being very bad when used for the body 



of a glower. Curve b shows the emission of a sample which has good 



electrical properties. It contained less than 0.01 per cent of silica, and 



was the purest obtainable sample. From this it is evident that the sharp 



maxima are not due to silica. The emission spectra are very similar and 



are conspicuous for two sharp emission bands, with maxima at 2.78/1 



(curve a, maximum at 2.83 //) and at 4.3 ft, respectively. Smaller maxima 



appear at 2, 2.4, 3.2, 4.7, and 5.4/*. From a comparison of these curves 



with those given in figs. 77 and 78 it appears that in the latter sample the 



"binder" used, probably yttria, has but little influence upon the sharp 



emission bands. 



Magnesium Oxide (MgO). 



(Curve c, fig. 80.) 



The magnesium oxide (curve c, fig. 80) spectrum shows two wide 

 bands at 3 and 5.3 n, respectively, with a smaller band at 2.1 it. Some- 

 what similar conditions were observed in fig. 70. 



In fig. 80, curve a gives the emission spectrum of the material used in 

 a Nernst glower placed upon a heater-tube. The general outline, and 



