NICHOLS. — THE VISIBLE RADIATION FROM CARBON. 103 



constant temperature in tbe rod, an assistant made repeated observa- 

 tions with the potentiometer and readjusted the resistance in the 

 battery circuit whenever necessary. Excepting at very higli tempera- 

 tures, where the rod was subject to rapid disintegration, it was rarely 

 necessary to make any adjustment during the progress of a single set 

 of observations. Readings of the current flowing through the carbon 

 and of the full of potential between its ends were made at the beginning 

 and end of each experiment. 



Spectrophotometric Observations. 



It was my expectation, in planning this research, that whatever might 

 prove true as to the character of the radiation from gray carbon, the 

 distribution of energy in the spectrum from black carbon would change 

 in such a manner with increasing: incandescence as to become nearly or 

 quite identical with that of the various luminous gas flames at tempera- 

 tures corresponding to the temperature of the glowing carbon iu those 

 flames. I had also hoped, among other things, to be able to bring about 

 a degree of incandescence approaching that of the acetylene flame itself, 

 before the usefulness of the thermo-eleraent as a means of measurinjj 

 the temperature failed because of the melting of the platinum wire, and 

 in this way to obtain a check upon my previous measurements of that 

 flame ; and at the same time to be able to determine the temperature of 

 any given luminous flame in which the incandescent material consists 

 of carbon particles by ascertaining the temperature of the carbon rod 

 for which its surface had a spectrum corresponding in distribution of 

 energy to that of the flame. 



It will be seen from inspection of the curves to be discussed in a 

 subsequent paragraph that this expectation was far from being realized, 

 and that the distribution of energy in the spectrum of the carbon rod, 

 instead of approaching that of the acetylene flame as the temperature of 

 the rod increased, took on an entirely unexpected character. Even at 

 low temperatures, that is to say up to about 1100°, the change in the 

 spectrum was not of the comparatively simple character which had been 

 anticipated, and shortly after passing the temperature of 1100°, unlooked 

 for complications in the results arose. The energy in the yellow of the 

 spectrum which from the beginning had been increasing at a relatively 

 more rapid rate than either in the red or at the blue end, became so 

 great as to give the distribution curve a form entirely contrary to 

 expectation. 



I was very slow to believe in the integrity of these results, and nearly 



