NICHOLS. — THE VISIBLE RADIATION FROM CARBON. 117 



1500°, does however not vary widely from a straight line. Beyond these 

 temperatures it is a matter of extrapolation, but the same thing is true 

 of all other attempts to estimate very high temperatures. The curve /, 

 for the relation of the logarithm of the intensities and the temperatures, 

 which I have also given in Figure 24 (between 1500° and 3750°), is in 

 the case of Lucas's measurements nearly straight ; so that in so far as 

 this is a criterion, his curve up to this point may be said to conform to 

 the Wien -Planck equation. It is significant that Lucas's curve shows an 

 inflection point between 3.300° and 4000°, becoming concave to the 

 axis of temperatures. This is the temperature at which, according to 

 nearly all the newer determinations, carbon, as in the crater of the arc, 

 approaches its maximum condition of incandescence. At about 3750° 

 the electrical energy developed in the rod is doubtless largely expended 

 in the disintegration or vaporization of the carbon, so that a maximum 

 degree of incandescence is approached. At the point at which this process 

 begins current can no longer betaken as a measure of the temperature. 

 The very slight falling off in the photometric measurement of intensity 

 does not appear to me to warrant the conclusion drawn by the author that 

 a maximum has been passed at the current value to which he assigns 

 the temperature 4750°. The difficulty of obtaining consistent readings 

 under conditions existing in such work would amply account for so slight 

 a discrepancy. 



Lucas's work appears, in a word, to warrant the following rather 

 important conclusions. First, that up to about 3750° current and 

 temperature in the case of carbon rods heated electrically are nearly 

 proportional. We have in favor of this point two checks, — the straight- 

 ness of the logarithmic curve and the fact that the inflection of Lucas's 

 curve corresponds, as has already been pointed out, to the recognized tem- 

 perature of the crater of the arc. Secondly, that for a wide range of 

 temperatures photometric intensity, like the intensity of total radiation, 

 follows the logarithmic law of iucrease. Third, that after the tempera- 

 ture of the crater has been attained a considerable additional increase in 

 incandescence results from the application of further current before the 

 maximum is finally attained. This agrees with the observations of 

 Moissan,* that many reductions in the electric furnace which do not 

 occur with moderate currents become possible by increase of the current 

 strength. If, as seems proper, we ascribe the rapid approach of Lucas's 

 curve to a finite maximum to the utilization of the energy of the cur- 



* Moissan, Comptes Kendus, CIX. 776 (1894). 



