﻿with Coloured Glasses and with the Spectroscope. 9 



3rd Period : 23J, 35, 43, 44, 44J, 45i 46, 47^, 48J, 50, 511, 

 52, 53, 56, 56i, 57, 61J, 62, 62J, 63, 65, 66J, 

 67, 67J, 70, 72, 100, 102, 103, 105, 108, 135. 



Among the dark bands detected,, the most intense occurred at 

 44-46, 51-55, 56-58, 62-64^; others were found at 33-34J, 

 361, 371^ 3 8 i, 40, 68-72. 



Many of the dark bands were crossed by bright lines. 



I have repeatedly observed the dark band considered by Roscoe 

 to be a hydrogen absorption-line, but have not noticed that its in- 

 tensity varied with the dampness of the weather. Whether it 

 is an absorption-band or not can be determined by a series of 

 observations continued through wet and dry weather. If this 

 proves to be a hydrogen-line, the Bessemer spectrum will be 

 found more complicated than is generally supposed. It has 

 been thought by some that the dark bands in the spectrum are 

 absorption-lines due to the cooling of the outer sheath of flame ; 

 but it is more probable that although the pellets of iron and 

 slag tend to produce a faint continuous spectrum, yet in con- 

 trast with the very brilliant lines it appears discontinuous, the 

 dark bands being merely intervals between the bright ones. 

 The iron spectrum has not been satisfactorily identified. It has 

 been suggested that the brightness and size of the lines of the 

 Bessemer spectrum do not allow the iron-lines to appear. In 

 comparing the Bessemer spectrum with Bunsen's spectra of 

 nickel, cobalt, and calcium, no coincidences were observed, except 

 two or three in the latter spectrum. The brightest calcium-line, 

 however, was not visible in the Bessemer spectrum. The Bes- 

 semer spectrum contains yet many mysteries to be solved, among 

 which is the cause of the non-appearance of the lines of the 

 spectrum at the beginning and termination of the blow. 



This was readily solved when the numerous lines of the spec- 

 trum were attributed to carbon; but in proving them to be 

 caused principally by manganese, their disappearance is not so 

 readily accounted for. 



One theory to account for it is that the luminous power of 

 the flame is too small at the beginning and end of the process 

 to produce a spectrum. In regard to this, it may readily be 

 shown that the brilliancy of the spectra of incandescent metallic 

 vapours does not depend upon the illuminating-power of a flame, 

 but upon the heat of the flame into which they are introduced. 

 For instance, the spectra are more distinct in the non-luminous 

 flame of a Bunsen lamp than in the ordinary luminous gas-flame. 

 If we take the theory as referring to the feebleness of light given 

 off by those substances in the flame which produce the spectrum, 

 it will resolve itself into the one of change of temperature, not- 



