Line Spectrum emitted by Iron Vapour. 287 



of an electric field. Furthermore, I have furnished experi- 

 mental evidence to the effect that the luminous vapour 

 produced in these spark discharges is not the result of a 

 vaporization of the electrodes by heat, but of some direct 

 action of the discharge current upon the molecules on 

 the surface of the electrodes*. Now in view of the fact 

 that a small quantity of luminous metal vapour, although 

 undergoing rapid cooling by a current of air, is capable of 

 emitting light radiations for a measurable time after the 

 exciting agent has ceased to act, should we not, by analogy, 

 actually anticipate a continuation of the line emission, 

 after the breaking of the electric current that caused it, in 

 the case of a vapour which is completely shielded from the 

 surrounding air by a slowly cooling furnace-tube ? With 

 his well-protected furnace Dr. King has been able to observe 

 some of the iron lines for as long as 5 minutes after breaking 

 the current. In my small furnace the luminous ladiations 

 die out much more rapidly, and the successive extinctions of 

 the various groups of lines is most interesting to follow. 

 The group of doublets at 4957 disappears first at i'rom 5 to 

 10 seconds after the current is broken. The strong con- 

 tinuous spectrum, which until then masks many of the lines, 

 begins now to clear, and at about 15 seconds after the 

 breaking of the current class I. groups y and 6 stand out 

 most conspicuously for a few moments on a dark background. 

 These changes present quite a beautiful spectacle. Thanks 

 to the relatively high luminosity of my spectrograph, it 

 has been possible to secure photographic records, with 

 exposures of only 1 or 2 seconds, at intervals of 10, 15, and 

 20 seconds after the interruption of the current. Further, 

 temperature determinations of the inner surface of the furnace 

 tube were made at corresponding intervals of time which 

 furnish some interesting data with reference to the rate of 

 cooling of the furnace. The values of these temperatures, 

 which are the means of two readings, are as follows : — 



Furnace temperature after an interval of 



Initial Furnace ^ * -s 



temperature. 10 seconds. 15 seconds. 20 seconds. 



2700° C. 2300° 2100° 2000° 



In an additional series of experiments the field was left on 

 partially, the current being dropped to about 180 amperes. 



* Hemsalech, Coviptes Rendns de VAcademie des Sciences, vol. cliv. 

 p. 872 (1912). 



