PORTER. — ON CERTAIN ARC SPECTRA. 379 



1. The average intensity of the iron and magnesium arcs is not 

 changed by substituting pure nitrogen for air as an atmosphere. The 

 average intensity of the zinc and tin arcs is reduced two or three times 

 by nitrogen. By hydrogen the average intensities are reduced from five 

 to twenty times as much as by nitrogen. Ammonia apparently does not 

 produce quite so great a reduction as does hydrogen. 



2. The relative intensity of many lines depends upon the atmosphere. 

 The lines that are relatively reduced by nitrogen are spark lines. As a 

 rule these lines are relatively enhanced by hydrogen or ammonia. 



3. The influence of ammonia on intensities and reversals is intermedi- 

 ate between that of nitrogen and hydrogen, and in general it seems true 

 that the effect of ammonia is approximately equal to the sum of the effects 

 of its constituents. This, in fact, is the particular point which I had in 

 mind to determine when I began these experiments. 



4. The influence of oxygen is similar to that of hydrogen. 



5. Nitrogen affects the reversed lines of tin by either destroying the 

 reversal or producing faint double reversals. 



These results seem sufficient to show that the readiness of an atmos- 

 phere to form chemical union with the electrodes under ordinary condi- 

 tions is a very small, probably insignificant, factor in determining the 

 intensity of the arc. The intensity appears to be due to electrical causes 

 rather than to chemical reactions. 



Some experiments have been performed by Professor Basquin* which 

 seem to confirm the theory that the intensification of spark lines in 

 hydrogen is caused by the increased resistance due to the hydrogen 

 atmosphere about the arc. This greater resistance has been attributed 

 to the absence of chemical reaction in the hydrogen arc. Such a 

 change of resistance in the products of the arc may explain the phe- 

 nomena which occur in hydrogen. But if the resistance of the arc 

 depends on the reactions in it this fact makes it difficult to see how the 

 spark lines can be intensified by an atmosphere of oxygen. 



This work was done under the direction of Professor Crew, who, in 

 fact, himself began it and to whom I am indebted for continued advice 

 and assistance. 



Northwestern University, Evanston, Illinois. 

 22 April, 1902. 



* Astroph. Jour., 14, 14-17 (1901). 



