BASQUIN. — THE ARC SPECTRUM OF HYDROGEN. 165 



Following the suggestion of Liveing and Dewar, above referred to, I 

 have tried the rotating metallic arc in air, playing a very small jet of 

 water upon the rotating electrode. In this manner the silver arc vv^orks 

 rather more poorly than usual, and resembles a rapid series of small 

 explosions. The hydrogen lines come out clearly, but are rather weaker 

 and more diffuse than in the hydrogen atmosphere. 



The copper arc works well in an atmosphere of steam, much better 

 than in hydrogen. The hydrogen lines are nearly, if not quite, as strong 

 in steam as in hydrogen. The electrodes of the arc are slightly oxidized 

 and have very beautiful colors. In making this experiment a slight 

 alteration was necessary in the hood of the arc. The window through 

 which the light issues is usually as far away from the arc as possible, but 

 it was moved for this experiment so as to be as close to the arc as pos- 

 sible. It was placed at the inner end of a brass tube projecting into the 

 hood, in order that the heat of the surrounding steam and hot air, as well 

 as that of the arc itself, might prevent condensation of steam upon the 

 surface of the window. 



CHEMICAL ACTION IN THE ARC IN HYDROGEN. 



Historical. 



Crew and Basquin * have sought to eliminate the radiations due to 

 chemical causes in the electric arc by using chemically pure metallic 

 electrodes and enclosing the arc in an atmosphere of hydrogen or nitro- 

 gen. They interrupted the current through the arc about 110 times per 

 second and examined the light of the arc while the current was null. 

 They found in the rotating metallic arc in air " a luminous cloud " ^qv- 

 sisting for several thousandths of a second after the current through the 

 arc had ceased, but they found no such luminous effect in an atmosphere 

 of hydrogen or nitrogen. This seems to show that the cloud is due to 

 chemical action going on in the gases after the electric current has 

 stopped, and that in hydrogen the chemical action is too feeble to be 

 noticed in this way. 



Liveing and Dewarf found a magnesium "line" at 5210, making its 

 appearance in the arc spectrum only upon the introduction of hydrogen 

 or coal gas into the arc. Professor Crew % gives a number of lines ap- 

 pearing in the iron arc in hydrogen and not appearing in the arc in air. 



* Proc. Amer. Acad., 33, 18 (1898). 

 t Proc. Roy. Society, 30, 96 (1880). 

 X Phil. Mag., 50, 497 (1900). 



