104 PROCEEDINGS OF THE AMERICAN ACADEMY. 



a year was spent in repetitions of the measurements before I could con- 

 vince myself that the {phenomenon was a genuine one. Measurements 

 taken upon a great number of different rods and at different times 

 showed the same result, however, and I was finally forced to the con- 

 clusion that the radiation from the carbon rods showed a much more 

 complicated law of distribution than had been anticipated, and that a 

 sort of selective radiation occurred such as to render the establishing 

 of any simple relationship between the curve of distribution and tem- 

 perature out of the question. 



The hope of being able to make direct temperature measurements up 

 to the melting-point of platinum was also disappointed. While the 

 carbon rods at comparatively low temperatures showed a fair degree of 

 stability under the action of the current, they appeared to undergo a 

 decided change of behavior at about 1400°, and before that temperature 

 a rather rapid disintegration, showing itself by a change of resistance, 

 manifested itself. This effect appeared to be similar to that which 

 shortens the life of the filauieuts of incandescent lamps when these are 

 subjected to a large amount of current. It appears, moreover, that at 

 these high temperatures the carbon tends to combine with the metals 

 of the thermo-element, affecting the electromotive force very much as 

 the vapors in a furnace have been found to do. The therrao-elements 

 inserted in the rod begin, in consequence of this action, to fail of their 

 purpose. It was found that after exposure to temperatures much above 

 1400°, the electromotive force corresponding to even lower temperatures 

 was considerably below the normal. I was consequently compelled to 

 abandon the attempt to measure directly temperatures above this point, 

 although it was possible to bring the rods to a higher degree of incan- 

 descence for a length of time sufHcient to perform the spectrophotometric 

 observations. In order to obtain at least an approximate estimate of 

 these temperatures, I made use of the fall of potential between the 

 terminals of the rod, and also of the current of the heating circuit ; and 

 by extending these curves, which, throughout the range of measured 

 temperatures were found to be nearly straight, to the high temperatures 

 which I wished to estimate, to obtain some idea, even if not an exact 

 one, of the latter. 



In expressing the results of the photometric measurements already 

 described, I have made use of two forms of curve. One set of curves, 

 in accordance with the nomenclature proposed in my original paper on 

 the visible radiation from platinum, and later adopted by Paschen and 

 other writers, I may call isotherms. These curves give in terms of the 



