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



lines as observed by Dr. de Watteville in the cone and 

 mantle of the air-coal gas flame *, and by me in the capacity 

 spark t, have been added for comparison. 



In addition to the line spectrum there also appeared 

 certain bands, especially the one with head line at 5007*3. 

 The bands are not affected in the fringe, but pass well down 

 from the plate into the protected space and, like the flame 

 line 4571, die oat gradually. 



The tabulated results bring to light a most important 

 fact — namely, the triplet lines, which are strongly emitted 

 under thermo-electrical excitation, are precisely those which 

 are likewise strongly emitted under chemical excitation in 

 the explosion region of the air-coal gas flame and under 

 electrical excitation in a capacity spark. Again, the only 

 line which from its character as a long line is evidently 

 a temperature line is also the only one which is emitted by 

 the mantle of the air-coal gas flame. In the capacity spark 

 it is a relatively feeble line, and appears, if at all, to be but 

 slightly affected by electric actions. The fact that the 

 triplets at 3838 and 5184, although very intense in the cone 

 of the air-coal gas flame, do not show in the mantle just 

 above the cone is probably due to the temperature of this 

 flame— namely 1850° 0., being too low for their emission 

 under thermo-chemical excitation. Even with the high 

 temperature of the furnace they only come out as relatively 

 faint lines below the fringe. There is no doubt that they 

 will show in the oxy-acetylene flame, but no data are as yet 

 available on this point. I have obtained them by simply 

 burning a lump of magnesium metal in air, although in 

 this case it is not quite certain whether and in how far the 

 emission of these lines is controlled by the heat generated 

 through combustion or by the chemical actions involved. 



That the triplet lines of magnesium are particularly 

 sensitive to electric actions will be further exemplified in a 

 subsequent communication by the fact that with a two-plate 

 furnace, in which a uniform electric field is established 

 between two parallel plates, these lines always appear first 

 at the cathode plate. 



No trace of the spark line 4481, or of any line of the 

 nebulous series, has been observed in the fringe or bluish 

 vapour. 



PL IX. a shows the ultra-violet triplet of magnesium in 

 the fringe ; the long thin tails of these lines which represent 

 those portions of the emission due to thermo-chemical 



* De Watteville, • Theses de Doctorat,' Paris, p. 34 (1904). 

 t Hemsalech, • Theses de Doctorat, Paris, p. 92 (1901). 



