294 Mr. G. A. Hemsalech on the Origin of the 



d. As in the electric spark, the line emission of iron vapour 

 in the furnace does not stop abruptly on the electric 

 field being removed, but continues for some time after, 

 and the extinction of the luminous vibrations is accom- 

 plished gradually, class III. lines disappearing first. § 4. 



<?. The spectrum emitted after the current is broken is not 

 controlled by the temperature of the furnace, as is 

 evidenced by the observation of class III. group 4957 

 at 2300° C. ; no trace of this group is seen in ordinary 

 circumstances even when the temperature of the furnace 

 has been raised to 2400°. § 4. 



/. It", instead of completely breaking the current, the latter 

 be only reduced to about 180 amperes, so that a feeble 

 potential gradient is left on, class III. group 4957 

 remains visible much longer, and it has been photo- 

 graphed when the furnace temperature had fallen to 

 2100° C. § 4. 



g. The absence of a line emission when the vapours of 

 copper, silver, and zinc are subjected to thermo-electrical 

 actions in the electric tube resistance-furnace at 2700°, 

 receives a satisfactory explanation by supposing that 

 they possess a low degree of electric conductivity 

 as compared with the vapours of iron, cobalt, and 

 nickel, which easily emit a line spectrum under the 

 same furnace conditions. This supposition is supported 

 by observations regarding the spark spectra of these 

 metals. § 5. 



h. The attempt to excite a line spectrum in iron vapour by 

 purely thermal actions in a furnace of special con- 

 struction has led to a negative result. § 6. 



i. A brilliant line spectrum of iron, similar in character to 



that observed in the tube-furnace at 2700°, was obtained 



with a new type of electric furnace in which a potential 



gradient of any desired strength could be established. § 7. 



II. As a result of my researches on flame and furnace 



spectra some light has been thrown on the various ways 



in which light radiations may be excited in iron vapour. 



For the sake of convenience, and also in order to facilitate 



the distinction between the several modes of excitation 

 which prevail in the flames, furnace, arc and spark, the 

 following denominations have been adopted : — 



a. Thermal excitation. By this is understood the emission 

 of luminous vibrations by the application of heat alone 

 in the absence of chemical or electric actions. No 

 line or band spectrum has been observed with iron 

 vapour. 



