290 Mr. G. A.. Hemsalech and the Comte de Gramont on 



In the arc they are generally confined to the neighbour- 

 hood of the poles. Examples : the iron lines XX3872 

 and 3936, and a number of " polar lines " observe'd by 

 Professor Duffield. 

 3rd type. — These lines are absent from flames and from the 

 tube furnace. In the arc they appear as relatively feeble 

 lines but are appreciably enhanced in the spark. 

 Examples : the iron lines XX 4924 and 5018, and the 

 cadmium line X 4416. 

 4:th type. — The characteristic spark lines. Examples : X 4481 

 of magnesium and XX 49 12 and 4924 of zinc. 

 More recently one of us observed that certain spark lines 

 which are absent from an arc passing between a carbon rod 

 and a metal electrode would show up well when the arc 

 is taken between two' big pieces of metal*. Further, with 

 the aid of visual observations of the spectrum he noticed 

 that whilst certain spark lines remained visible throughout 

 the duration of the arc and could thus be photographed 

 along with the arc lines, others flashed up only momentarily 

 at the beginning when the arc is struck. He thus 

 established the existence of two types of spark lines emitted 

 by the arc, namely : 



1. Lines which are permanently emitted. 



2. Lines which appear only momentarily at the anode on 



striking the arc. 

 Experiments made with plate furnaces raised to over 

 3000 ° 0. — thus approaching the temperature of an arc but 

 in which the acting electric fields were relatively feeble — 

 had failed to reveal the least traces of spark lines in the 

 case of magnesium t- 't was therefore thought that possibly 

 a high degree of ionization, such as obtains in an electric 

 furnace, would prevent the excitation of spark lines by 

 opposing the establishment of high potential gradients. 

 For it seemed to us that the electrical conductivity of the 

 metal vapours at the moment of striking the arc, as also of 

 those in an arc which passes between voluminous lumps of 

 metal possessing a big thermal capacity and a large cooli no- 

 surface, would be appreciably lower than that of the well 

 protected vapours in a high temperature furnace. Stimulated 

 by these considerations we decided to make a series of 

 experiments with the object of ascertaining the relative 

 roles played by thermal, chemical, and electric actions in the 

 emission of spark lines by an electric arc. Accordingly, our 

 observations bear more specially upon the effect of cooling 



* A. de Gramont, Comptes Bendus de V Academie des Sciences, clxx. 

 p. 31 (1920). 



t Hemsalech, Philosophical Magazine, vol. xl. p. 308 (1920). 



