228 Mr. G. A. Hemsalech : A Comparative Study 



2. An iron spectrum has been observed at the low temperature 



of 1500° C, the spectrum being the same as that given 

 by an air flame burning in coal gas. § 6. 



3. The spectra emitted by iron compounds in flames are 



identical with those given by the furnace at corre- 

 sponding temperatures up to about 2400° C. From this 

 and other considerations it has been concluded that the 

 mode of excitation must be the same in the two cases, — 

 namely, chemical dissociation of an iron compound by 

 the action of heat. §§ 4, 5, 6. 



4. The character of the spectrum is independent of the nature 



of the iron compound, which is acted upon by the thermal 

 forces in either flame or furnace ; thus chlorides, oxides, 

 &c. always give the same kind of spectrum in either of 

 these sources at a given temperature. § 4. 



5. The name thermo-chemical excitation has been adopted in 



order to designate the cause of emission of these spectra 

 both in flame and furnace. They differ completely from 

 the spectra given by the same compounds in the explosion 

 region of the air-coal gas flame where the emission is 

 due to chemical excitation at a comparatively low 

 temperature. §§ 4 & 7. 



6. A new group composed of class I. lines and possessing 



similar character as group 7, has been found with head 

 line at \5270. § 8. 



7. The aluminium lines XX3944 and 3962 have been observed 



at so low a temperature as 1500° C. § 9. 



§ 14. Concluding Remarks. 



Without entering into a fruitless discussion concerning the 

 mechanism involved in the generation of the atomic vibration s, 

 I hope, however, that a useful purpose will be served by 

 briefly considering the possible changes in the state of the 

 compound molecule when it is subjected to thermal actions. 

 From all the evidence to hand it appears to me doubtful 

 whether the iron atom in the compound is actually liberated 

 either in the furnace or in flames. Thus, for example, if we 

 heat iron oxide in a ilame the product would hardly con- 

 stitute a mere mixture of oxj^gen gas and iron vapour. 

 I rather believe that in the particular case under consideration 

 chemical affinity exerts its force even up to the temperature 

 of 2700° C. and that the compound molecule, although un- 

 doubtedly changed in so far as the relative positions and the 

 orbital motions of its component atoms are concerned, is not 

 broken up, but retains its individuality throughout. The 



