EFFECT OF PRESSURE UPON ARC SPECTRA. 
123 
PAET II. 
The Effect of Pressure upon the Spectrum of the Iron Arc. 
1. Preliminary. 
Most intimately connected with the j>roblem of the influence of pressure upon arc 
spectra are the names of Humphreys and Mohler, # who in 1897 published a full 
account of their investigations in this subject. They formed an arc between metal or 
cored carbon poles within a strong metal chamber furnished with a window, through 
which the arc was observed. Air or other gases could be pumped into the apparatus, 
and it was found that with an increase of pressure the spectrum underwent a change, 
the lines in general becoming broader and being slightly displaced towards the region 
of longer wave-lengths. Up to 14|- atmospheres, which was the highest pressure 
reached, the displacement of the lines for all the metals investigated was found to be 
directly proportional to the pressure, and a subsequent photograph of the spectrum 
of iron, which Humphreys obtained at 37 atmospheres in 1905,t tended to confirm 
this. ? 
Mohler| extended the work to pressures lower than one atmosphere, and the 
linear relation was still found to hold. In 1906, Anderson § obtained a photograph 
of the iron arc at a pressure of 30 atmospheres. 
The present research was begun by the writer in November, 1.904, with the object 
of extending the work of Humphreys and Mohler to still higher pressures, and two 
sets of photographs of the direct-current iron arc have now been obtained under 
pressures ranging from 1 to 100 atmospheres, the surrounding medium being in all 
cases air. This work was greatly facilitated by the preliminary work of Hutton and 
Petavel,|| who in 1903 published the results of a research made with the same 
pressure apparatus and a 1-m. Rowland grating. The possibility of photographing 
the spectrum from an arc under pressures up to 100 atmospheres was demonstrated, 
as well as the probability of a successful use of the pressure apparatus in conjunction 
with a spectroscope of the highest dispersive power. 
In August, 1906, the writer % exhibited to the British Association photographs of 
the arc spectrum of iron taken with the 2lUfeet grating under pressures varying 
from 1 to 101 atmospheres. 
* Humphreys and Mohler, ‘Astrophysical Journal,’ VI., 169, 1897 ; also ‘ Astrophysical Journal,’III., 
114, 1896 ; IV., 175, 1896; IV., 249, 1896; for data published during the progress of their work. 
t Humphreys, ‘Astrophysical Journal,’ XXII., 217, 1905. 
1 Mohler, ‘ Astrophysical Journal,’ IV., 175, 1896. 
§ Anderson, ‘Astrophysical Journal,’ XXIV., 221, 1906. 
|| Hutton and Petavel, ‘Phil. Mag.,’ 6, p. 569, 1903. 
*i Duffield, ‘Brit. Assoc. Report,’ York, p. 481, 1906. 
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