of the Flame and Furnace Spectra of Iron. 229 



luminous vibrations set up by the iron atom forming part of 

 a compound should, if my view were correct, show signs of 

 the effects of the chemical forces which bind the atoms in the 

 molecule. Now it has been shown in the course of this 

 investigation that the line spectrum emitted by an iron 

 compound at a temperature of 2700° C, although exceedingly 

 intense, is nevertheless most appreciably restricted in range, 

 it being composed almost entirely of class I. and II. lines 

 only, as though some extraneous force were preventing the 

 natural development of the luminous vibrations. This cur- 

 tailment of the luminous vibrations could be satisfactorily 

 accounted for if we accept the view expressed above, namely 

 that the iron atom, even at the temperature of 2700°, is to 

 some extent still associated with the other atoms in the 

 compound. On the other hand, it is legitimate to assume 

 that under the action of the powerful electric forces prevailing 

 in the arc or condenser spark, the iron atoms are really set 

 free and therefore enabled to execute their vibrations 

 without restraint. This assumption seems to be amply borne 

 out by the very high degree of development which cha- 

 racterizes the spectra emitted by iron vapour in these sources 

 of light. 



In this connexion it is interesting to compare the spectrum 

 of the high temperature oxy-acetylene flame with that given 

 by the low-temperature Bunsen cone. It will be remembered 

 that I explained the origin of the cone emission by assuming 

 the existence of a strong chemical affinity between the metal 

 and the nitrogen, resulting in the formation of a nitride. 

 Now if this hypothesis were well founded the iron atom 

 would leave its partners in the original compound, which is fed 

 into the flame (oxide, chloride, &c), and join that of nitrogen. 

 Hence, during the process of changing partner, the iron 

 atom may be conceived to be quite free for a short moment 

 and thus to be capable of executing its proper vibrations 

 without hindrance. We should therefore expect, in accordance 

 with the views put forth above, that its spectrum in this case 

 would be better developed than in any of the high-tempe- 

 rature flames up to 2700° C, in all of which the iron atom 

 is supposed never to be completely liberated from its partner 

 in the compound. This conclusion is indeed substantiated 

 by the facts observed, for the very lines which appear as 

 mere traces with thermo-chemical excitation at 2700° C. 

 stand out plainly in the spectrum of the cone emission, which 

 as regards development approaches that of the self-induction 

 spark. 



Conversely, we may assume that at the lower flame and 



