BASQUIN. — THE ARC SPECTRUM OF HYDROGEN. 171 



cannot originate the arc, and the chemical cause follows the electrical in 

 point of time, as is shown by the " luminous cloud " of Crew and Basquin 

 above referred to. The thermal cause also must depend upon the electri- 

 cal cause in some way. It probably depends upon it directly, but in any 

 event, it is a function of it through the chemical cause, for all chemical 

 reactions either take in heat or give off heat. 



Let us consider two arcs which are alike except that a larger current 

 runs through the first than through the second. Since the secondary 

 causes of radiation go hand in hand with the electrical cause we may 

 expect the first arc to have a spectrum which is uniformly brighter from 

 one end to the other than that of the second arc. With the exception of 

 a slight variation probably clue to conduction losses, this is just what is 

 always observed and confirms the secondary character of the chemical and 

 thermal causes of radiation. If these causes were not dependent upon 

 the electrical cause, we might possibly get an arc which would give only 

 a flame spectrum or an arc which would give only a spark spectrum. 



Let us now suppose that we run the same current through both the 

 similar arcs, and suppose that in some way we reduce the chemical action 

 going on in the second arc. What difference may we expect to observe 

 in them ? 



A reduction of the chemical action necessarily involves a reduction of 

 the temperature of the arc, because the chemical reaction in the arc in air 

 is exothermic, We have then an arc of lower temperature. If it is a 

 stationary arc it will be shorter and will go out more frequently. If it 

 is rotating it will have a smaller flame and work more poorly. All of 

 which is amply verified by experiments in hydrogen. 



But we may expect this reduction of chemical action to have certain 

 effects upon the spectrum. If all the lines of the spectrum of this arc 

 were functions of the electrical cause alone, then there would be no re- 

 duction in intensity of any part of the spectrum when the chemical action 

 is reduced. Professor Crew estimates from 5 to 100 times as the reduc- 

 tion in intensity caused by the hydrogen atmosphere. The electrical 

 cause alone can account, then, for only a small part of the radiation. 

 The secondary causes play very important parts. 



If all the lines of the spectrum of this arc were the same function of 

 the causes of radiation, then all the lines of the spectrum would be 

 uniformly reduced in intensity upon the reduction of chemical action. 

 Experiment shows this hypothesis to be too broad, but the lines belong- 

 ing to the series of Kayser and Runge are uniformly reduced in intensity, 

 so that it is probable that these lines are all the same function of the 

 causes of radiation. 



