EFFECT OF PRESSURE UPON ARC SPECTRA.- SILVER. 41 



per unit of area of the plate be considered, it seems true that the members of the 

 1st sub-series lines a and c, Plate 1, have diminished in intensity relatively to the 

 other two silver lines. Though if the whole intensity of the radiation in the region 

 of the plate occupied by the broadened lines be considered, the relative strengths of 

 the lines are very difficult to determine. 



Under pressure, all the lines in this part of the spectrum become obliterated. It 

 may be here remarked that the complete obliteration recorded by the photographs 

 with the large Rowland grating are not entirely confirmed by photographs with a 

 1 m. grating. These latter sometimes show faint indications of lines under high 

 pressures where none are apparent in those taken by the 2l feet instrument. This 

 is due to the fact that the lines do appear in the spectrum under pressure at the 

 moment of extinction of the arc, and possibly at the moment of striking it 



(c/. p. 44). 



With large dispersion the duration of these lines is insignificant compared with the 

 total length of exposure necessary to affect the plate, but when the dispersion is 

 smaller the exposure is less, and the interval during which the lines shine out is 

 comparable with the total exposure necessary. 



The lines to vanish first are those of the 1st subordinate series (a, X = 4055'44 

 and c, X = 4212 '!), which are of great intensity at normal pressure. In this respect, 

 and also because they become dissipated before they are obliterated, their behaviour 

 is very similar to the 1st subordinate series lines in the corresponding region of the 

 copper spectrum, 4022 and 4063. 



The line of the 2nd subordinate series (e, \ = 4476'29) weakens as the pressure 

 is increased, and is only just visible at 40 atmospheres. It has disappeared at 

 60 atmospheres. The manner of its disappearance allies it to the two lines in the 

 copper spectrum, also members of the 2nd subordinate series (4480'6 and 4531 "0), which 

 also become weaker as the pressure is increased, but without abnormal widening. 



The non-series line (d, X = 4311 "28) is clearly seen at 25 atmospheres, and at 

 40 atmospheres it seems to be the violet head of a band, but it is very doubtful 

 line' and the band are in any way causally related-photographs at higher pressure 

 suggest that the band is independent of this line, which has at 60 atmospheres cea 

 toexist. This non-series line differs from the non-series copper lines 

 examined; the latter have not been found to undergo complete obliteration, t 

 the " sharp" series are weakened as the pressure is increased. 



9. Series of Lines in the Silver Spectrum.-The classification of the lines of the 

 silver spectrum has been attempted by the same methods that have 



for the copper spectrum. 



KAYSEB and RUNGE'S* classification is based upon the relationships between 

 frequencies of the lines, and the series have become known as the 



* KAYSEU and RUNGK, ' Uber die Spectren der Elemente,' vol. V., 1892. 



VOL. CCXI. A. 



