PORTER, — ON CERTAIN ARC SPECTRA. 375 



exposure with the arc in air. The exposures were so timed as to make 

 the intensities of the two inner photographs intermediate between the 

 intensities of the two outer ones. By comparison one can readily deter- 

 mine from such a plate whether any change in intensity is due to length 

 of exposure or to change of atmo^nhere. This plan of making four 

 exposures on each plate has some additional advantages: (1) it affords a 

 test of uniformity of results ; (2) the two short exposures allow an easier 

 comparison of lines so strong as to be ordinarily over-exposed ; (3) the 

 second exposure in the gas being made immediately after the first, and 

 without opening the " hood " of the arc, a photograph is obtained of the 

 spectrum of the arc in its final atmosphere ; that is, in an atmosphere 

 which includes the gaseous products of the arc, if there be any. 



The effect of hydrogen was taken from the results published by 

 Professor Crew * and from the original plates obtained by him. These 

 necfatives were made under the same conditions as here described, 

 except in a different atmosphere and with a direct current. The change 

 from a direct to an alternating current, however, produces no effect on 

 the spectrum. In the case of the tin arc less dust is produced when a 

 direct current is used, and I have therefore employed such a current in 

 photographing the spectrum of tin in ammonia. 



The hydrogen atmosphere was obtained by the electrolysis of acidu- 

 lated water. The ammonia used was taken from a drum of compressed 

 ammonia gas such as is used in refrigeration. Nitrogen was generated 

 by the reaction of ammonium sulphate and sodium nitrite solutions. 

 Professor J. H. Long kindly suggested an arrangement of the nitrogen 

 generator by which air was excluded from the entire system throughout 

 the work.. 



Traces of oxygen were removed by pyrogallic acid, while the water 

 vapor was taken out by passage through concentrated sulphuric acid and 

 over phos})horus pentoxide. 



Before commencing an exposure a strfeam of nitrogen was kept flow- 

 ing through the hood for at least twenty minutes. The same plan was 

 used in filling the hood with ammonia. During au exposure a stream of 

 the gas was kept flowing through the hood, both for the purpose of 

 keeping a fresh supply of the gas about the arc and to drive out the dust 

 which formed. 



The spectra of the following four metals have been examined 

 photographically in the region lying between X 2300 and A 5300. 



* Astropli. Jour., 12, 167-175 (1900). 



