NERNST GLOWER ISOCHROMATICS. 1 27 



these two points, as previously observed. This method of locating the 

 maximum eliminates the correction for slit-width, and is an independent 

 proof of the previous observations that the maximum of the energy curve 

 for normal burning (80 watts) can not lie at such short wave-lengths as 

 was observed by previous investigators. This, of course, is on the assump- 

 tion that the glowers were of the same material, the base of which is zir- 

 conium oxide with a small per cent of thorium or yttrium oxide. 



In Carnegie Publication No. 35, p. 318, it was shown that in the case 

 of vacuum-tube radiation the intensity of the emission lines is proportional 

 to the energy consumption. The graphs there obtained, showing the 

 relation between current and emissivity of a spectral emission line, are 

 curved, due to the fact that Ohm's law does not hold for the vacuum-tube 

 discharge. For the present examination, instruments were available to 

 measure the energy consumption, when the graph ought to be a straight 

 line, provided the partition of the energy emitted is in discrete lines. As 

 a typical, selectively radiating solid, oligoclase was chosen on account of 

 its sharp emission bands, and also on account of its homogeneity. A rod 

 2.5 cm. long and 2 mm. diameter was prepared in an oxyhydrogen flame. 

 The spectrometer slit was reduced to 5 mm. in length, which permitted 

 the entrance of radiation from only about 5 mm. of the central part of the 

 rod, which was a perfectly clear glass, free from air-bubbles. At the 

 highest temperatures this central part showed a peculiar faint white "lumi- 

 nescence" similar to the intense white noticeable in quartz when heated in 

 the oxyhydrogen flame. The rod was thickened at the ends which seemed 

 to prevent internal reflection of the radiation from the platinum terminals. 



The distribution of energy from the central portion of this rod is shown 

 in curve c, fig. 66, on 29.4 watts. The ends of the 0.3 mm. platinum 

 terminals within the glass rod were red hot. The rod was viscous, indi- 

 cating a temperature of at least noo to 1200 ; but no light was emitted 

 other than the hazy white glow already mentioned, which is in marked 

 contrast with the radiation from the platinum electrodes. A similar 

 example is given in Wood's Optics, page 457, where it is stated that sodium 

 sulphate, in a loop of platinum wire, heated in a blast lamp, emits but 

 little light, although the wire glows vividly. 



In fig. 96 are given the isochromatic emission curves of oligoclase at 

 wave-lengths 2.048, 2.905, 4.445, an d 6.082 //, respectively, for different 

 values of power consumption. The graphs are for the same rod, under 

 the same conditions of galvanometer sensibility and distance of the radiator 

 from the silt. Two additional series of observations on different days, 

 and using different adjustments, were made at wave-length 2.905 fx. Only 

 one of these graphs was parallel with the one given in fig. 96, showing that 

 there is a variation in the slant of the isochromatic curve, under different 

 conditions. It will be noticed that, throughout the range investigated, the 

 change in emissivity is proportional to the energy supplied, just as is true 



