of the Flame and Furnace Spectra of Iron. 217 



light radiations coming from the centre of the furnace, where 

 we may rightly assume them to be of maximum intensity for 

 a given temperature, have to pass in the present case through 

 -at least 2 inches of vapours, of which the last inch or so is 

 probably well below the temperature prevailing at the centre. 

 In flames the radiations do not traverse such a thick layer of 

 cooler vapours, for practically the whole of the active volume 

 of radiating vapour is confined within the limits of the flame 

 envelope, which in the high-temperature flames rarely exceeds 

 one centimetre in diameter. This fact no doubt accounts for 

 the flame spectrum being so much better developed in the 

 ultra-violet than the furnace spectrum, and it also explains 

 the absence of reversals in the former. On the other hand, 

 the furnace seems to be more efficient as regards line-intensity 

 in the visible part of the spectrum, as though the longer 

 wave-lengths were less absorbed than the short ones. This 

 might indeed be the correct explanation, for the column of 

 radiating vapour, even in my small furnace with an active 

 length of 4 inches, must extend to at least 2 inches, as com- 

 pared with an active depth of only from 5 to 10 mm. in 

 flames. Hence, in judging the results of intensity estimations 

 in flame and furnace spectra, account should be taken of the 

 above considerations, and, further, it should always be re- 

 membered that it is not the real intensity of a line which 

 counts, but its relative intensity and, particularly, the relative 

 behaviour in each source of the various definite groups of 

 related lines. The estimation of line intensities would, in the 

 presence of a strong continuous background, be on a lower 

 scale throughout than when this disturbing factor is absent. 

 Thus it would be of no consequence if, for example, both 

 group 7 and the triplet at X 4384 were observed to be intrin- 

 sically brighter in the flame specirum than in the furnace 

 spectrum at the same temperature, provided that the relative 

 intensities of the triplet lines with regard to those of group 7 

 be the same in the two cases. Hence in using the compa- 

 rative table of flame and furnace spectra, which has been 

 established as a result of my intensity estimations, these 

 recommendations should be borne in mind, and attention be 

 directed more specially to the relative intensities of the lines 

 in any one particular spectrum, than to the real intensities of 

 the lines in one spectrum as compared to those of the same 

 lines in another spectrum. 



The scale of intensities adopted is the same as that outlined 



in a former communication*. All wave-lengths are expressed 



in international units. A horizontal bar — means that no 



line has been observed. Ad or tr attached to the wave-length 



* Hemsalech, /. c. I. p. 9. 



