of the Flame and Furnace Spectra of Iron. 227 



§ 12. Explanation of Plate X. 



The photographs here reproduced are enlarged copies 

 (4 times) of the visible portion of the iron spectrum obtained 

 under various conditions of excitation. Nos. 1—5 sbow the 

 development of the iron spectrum at various stages of tem- 

 perature both in furnace and flames. At the higher furnace 

 temperatures, namely 2100° (No. 3) and 2400° (No. 4) a 

 strong continuous spectrum impedes the distinctness of the 

 iron lines. But if, as has been explained in § 5, attention be 

 directed more particularly to the relative behaviour of class I. 

 group 7 and class II. triplet at 4384, it is easy to see that 

 the latter gains steadily and continuously in relative intensity 

 as the temperature rises. Thus at 1600° (No. 1) the line 

 4384 is much weaker than its neighbour 4376; at 2550° 

 (No. 5), on the other hand, it is considerably brighter than 

 4376. No. 6 shows the spectrum of iron as emitted in the 

 explosion region of the air-coal gas flame. This spectrum 

 was obtained with burner No. 1 and the method of screening 

 already described*. The most striking feature of this 

 spectrum, although emitted at a temperature of less than 

 1700° C, is the presence of class III. group at 4957, which 

 is entirely absent from the spectra given by flames, or the 

 furnace up to 2500° C. As already mentioned in § 6, a mere 

 trace of some of the lines in this group is observed in the 

 oxy-acetylene flame at a temperature of 2700° C. In the 

 explosion region, as will be seen from an inspection of No. 6, 

 the lines in this group are quite as intense as those of 

 class I. group 7. Class II. triplets at 4272 and 4384 are 

 likewise relative^ very bright as compared with group y. 

 The last spectrum, No. 7, was obtained with the self-induction 

 spark, and represents an example of electrical excitation. It 

 is characterized by the outstanding prominence of the groups 

 of class II. and III. lines. 



§ 13. Summary. 



1. The spectra of iron as given by an electric-tube resistance 

 furnace at atmospheric pressure and up to a temperature 

 of about 2400° C, are caused by the action of heat on a 

 chemical compound of the metal and not on the free 

 metal itself. Hence these spectra are not of purely 

 thermal origin. § 4. 



* Hemsalech, /. c. I. pp. 9 & 10. 

 Q2 



