306 Mr. G. A. Hemsalech on the Luminous Vapours 



upon the latter, and passing much farther from the plate, is 

 the spectrum caused by the action of heat upon chemical 

 compounds of the elements contained in the substance under 

 examination (thermo-chemical excitation). The character 

 of a light radiation can then be derived from the relative 

 degree of its response to these two modes of excitation as 

 indicated by the corresponding line intensities in the fringe 

 and in the luminous vapour. 



In order to facilitate the grouping of the lines according 

 to character, the following three standard types based upon 

 their relative appearance in fringe and luminous vapour 

 have been adopted : — 



First type. Long lines. These constitute the so-called flame 

 lines, which are emitted by the mantles of the various 

 low- and high-temperature flames. They are caused by 

 the action of heat upon chemical compounds, and their 

 intensities are controlled by the plate temperature. 

 The thermelectronic current does not appear to affect 

 these lines appreciably, and they generally pass far down 

 from the under surface of the plate through the pro- 

 tected space, fading away gradually. 



Second type. Tadpole lines. These are specially sensitive 

 to the thermelectronic current in the fringe and to 

 a lesser degree also to thermo-chemical actions in the 

 luminous vapour. On account of this peculiarity these 

 lines appear to consist of an intense head, to which is 

 attached a long thin tail. They are strongly emitted in 

 the explosion region of the air-coal gas flame, but are 

 either absent from, or appear only as mere traces in the 

 hottest zone of this flame just above the cone. They 

 are generally brought out in the mantles of high- 

 temperature flames. 



Third type. Short lines. The emission of these lines is 

 almost entirely confined to the fringe : only faint traces 

 are in some cases observed under thermo-chemical 

 excitation at very high temperatures. They are, how- 

 ever, easily emitted under chemical excitation in the 

 explosion region of the air-coal gas flame. 



This classification naturally only serves as a guide in the 

 establishment of the most probable character of a line, as in 

 some cases it is difficult to decide into which category a line 

 should be placed. For this reason the relative intensity of 

 each line has been recorded both for the fringe and the 

 region just beneath it. 



