226 Mr. G. A. Hemsalech : A Comparative Study 



§ 11. Relative Merits of Flame and Furnace as a means 

 of obtaining the Spectrum of Iron by Thermo-cliemical 

 Excitation. 



Although the spectra given by the furnace are of the same 

 character as those observed in flames, there are occasions 

 when it is of advantage to give preference to either one or 

 the other of these two light sources. Thus the furnace, for 

 the reasons already suggested (§ 5), gives a better developed 

 spectrum in the visible part. On the other hand, all the 

 flames are superior to the furnace in the ultra-violet, where 

 the latter absorbs a large percentage of the radiations. Also, 

 with low-dispersion apparatus the flames give a purer 

 spectrum of the metal owing to the relatively much smaller 

 range of the continuous background. Moreover, the oxy- 

 acetylene flame provides a means of investigating the effects 

 of thermo-chemical excitation at a much higher temperature 

 (2700°) than is possible with the tube furnace, since the 

 latter exhibits a totally different phenomenon above 2500° C. 

 Again, from the point of view of manipulating these sources 

 of light, the flames possess this further advantage that their 

 temperatures can be kept constant for any length of time 

 without the least trouble, whereas the furnace has to be con- 

 tinuously watched and readjusted, because the gradual dis- 

 integration of the walls of the carbon tube entails changes in 

 resistance and, consequently, in temperature. And, last not 

 least, the spectra given by flames are free from the countless 

 impurities which infect every furnace spectrum. On the 

 other hand, the furnace permits to carry out the experiments 

 at low or high pressures, as has been so effectively done by 

 Dr. King, whereas flames can not be so conveniently sub- 

 jected to such experimental variations. 



With regard to the practical working of the furnace for 

 spectroscopic purposes, it seems to me that improvements in 

 several directions are possible. Thus the spark method, 

 which has proved so successful with the high-temperature 

 flames, might with advantage be applied to the furnace. 

 Also the Grouy sprayer should give satisfactory results. 

 Another method of obtaining the furnace spectra of metals 

 would be to add these latter to the carbon or graphite from 

 which the furnace tubes are made and turn them into 

 carbides. With the disintegration of the tubes, these carbides 

 would be set free and exposed to the thermal actions in the 

 furnace. 



