406 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



K Fe K 



T 



Phase t\ second : merest trace of Fe line. 



Phase I second : Fe line distinct. 



Pliase 1 second : Fe line lialf as strong as side line. 



Phase 1;^ seconds : Fe line still the weakest one in the triplet. 



Phase 1| seconds : Fe line equal to weaker K line. 



Phase 2| seconds : Fe line equal to stronger K line. 



Phase b\ seconds : Fe line strongest one in triplet. 



Phase 7| seconds : K lines very weak, but distinct. 



Phase 12 seconds : merest trace of K lines. 



Cold spark: A' lines invisible. 

 Figure 6. 



potassium pair diminishes with the temperature. In this comparison 

 the three cyanogea bands are taken as the standard of intensity, and 

 liave essentially the same density on each plate. The triplet thus 

 assumes the successive appearances shown in the accompanying figure. 

 If we had measured the temperature of the region between the carbon 

 poles at each of these nine phases, we could have identified with certainty 

 each of these temperatures from the appearance (relative intensity) of 

 the triplet. It is not to be forgotten that the temperature here referred 

 to is not the much-talked-of and little-understood " temperature of the 

 spark ; " nor is it any temperature peculiar to certain " streaks " as 

 perhaps is the case in the Geissler tube discharge. The temperature 

 here referred to is that of the medium at the instant in which the shutter 

 of the spectrograph is opened. The appearance of this triplet is then 

 a criterion for a temperature which may be measured directly with 

 a thermo-electric couple of sufficiently fine wire ; it is a function of the 

 phase, and not of the duration, of the exposure. 



NORTHWESTEKN UnIVERSITV, EvANSTON, ILLINOIS, 



July 19, 1902. 



