220 Mr. Gr. A. Hemsalech : A Comparative Study 



number indicates that the line is double or treble, but has not 

 been resolved by my spectrograph ; in all these cases the 

 mean value of the wave-lengths o£ the components has been 

 given. An r attached to the intensity number signifies that 

 the line is reversed. The class and therefore the character 

 of each line is given in a separate column in accordance 

 with the classification which L have proposed in the paper 

 referred to. The numerical results are arranged in order of 

 ascending temperatures, irrespective of source ; this arrange- 

 ment has the advantage of enabling the reader to follow in a 

 convenient manner the gradual development of the spectrum 

 of iron as the intensity of the thermal actions on the com- 

 pounds involved increases. 



§ 6. Remarks on the tabulated Results. 



The emission of lines begins at the remarkably low tempe- 

 rature of 1500° C, and the spectrum, which consists of about 

 seven lines, is practically identical with that given by iron in 

 an air flame burning in an atmosphere of coal gas*. Thus 

 already from the first signs of response to the thermal 

 actions the luminous vibrations set up by the iron atom, 

 both in flame and furnace, are of the same character. The 

 next higher temperature, namely 1600° C, marks an inter- 

 esting stage in the development of the iron spectrum, for at 

 this point class I. quintets y and e (see § 8) form the most 

 prominent feature in the visible part and the grouping of 

 the lines is strikingly revealed. As the temperature rises the 

 number of lines increases, as does also the brightness of the 

 spectrum, but some lines gain more rapidly in intensity than 

 others. The spectrum of the air-coal gas flame at 1850° C. and 

 that of the furnace at 1900° C. are practically identical ; so 

 are also the spectra given by the furnace at 2400° C. and the 

 oxy-coal gas flame at 2450° C. It will be noticed that in a 

 number of cases feeble lines appear relatively more intense 

 in the furnace spectrum than in the corresponding flame 

 spectrum. This might be due to the fogging of the plate 

 caused by the continuous spectrum always present in the 

 furnace emission ; for, as Professor R. W. Wood has shown, 

 faint impressions on a photographic plate always show up 

 in a remarkable manner when the background is slightly 

 fogged. 



The effect of temperature on lines of different character is 

 well illustrated by the relative behaviour of class I. group y 



* Hemsalech, I c. II. § 8, p. 233, 



