EFFECT OF PRESSURE UPON ARC SPECTRA. GOLD. 59 



care with which the grating had been adjusted, it was possible to move from one 

 p.-irt of the spectrum to another and still have excellent definition, without further 

 .nljiistiiient than that involved in making a correction for the temperature of the 

 room, which is different in different parts of the spectrum and which had been 

 previously found experimentally and tabulated for subsequent use. * 



Milking allowance for some overlap upon the neighbouring photographs each plate 

 contributed about 600 A.U. The photographs (Table IT., p. 58) were taken in the 

 regions indicated at the subjoined pressures. 



The pressures are the excess above 1 atmosphere. In each region a photograph 

 was also taken of the spectrum of the gold arc at atmospheric pressure in conjunction 

 with that of an iron arc. 



(2) Description of the Plates. Plates 2, 3, and 4 illustrate the behaviour of the 

 gold arc under different pressures : Plate 2 includes the region X = 3770 to X = 4020- 

 Plate 3 the region X = 4010 to X = 4280, and Plate 4 the region X = 4300 to 

 X = 4620. The photographs are full size positive reproductions of the originals, and 

 are arranged in order of increasing pressure from the top at 1 atmosphere to the 

 bottom at + 200 atmospheres, the central strip being always at atmospheric pressure. 

 The arbitrary letters enumerated in Table I. have been affixed to the lines to facilitate 

 reference to them. 



The prominent features of the gold arc are : 



(1) The broadening of the lines. 



(2) Their displacement towards the red end of the spectrum. 



(3) The changes in relative intensity. 



(4) The absence of reversals. 



The Nitrogen Band Spectrum. Upon the photographs reproduced in Plates 2 

 and 3 occur at 10 atmospheres pressure two band spectra, one whose head is a 

 little to the violet side of g2 (Plate 2) being very well marked. Measurement gives 

 3914'45 and 4278'28 as the wave-lengths of their chief heads, and they are evidently 

 due to nitrogen, the former coinciding with the nitrogen band measured at 39 14 "60 

 by DfiSLANDREst and the latter with that at 4278'6 measured by HASSELBERO.J 

 LEWIS and KING have already pointed out that these bands are liable to occur 

 in the arc spectra of some metals, especially of copper, and this is evidently an 

 instance. 



Both bands are ascribed to the negative band spectrum of nitrogen. 



The silver spectrum under pressure was characterised by the production of a band 

 spectrum of quite a different nature from the above, each band being a broad 



* W. G. DUFFIELD, 'Phil. Trans.,' A, vol. 208, p. Ill, 1908. (Part I. The Mounting of the Rowland 

 Grating.) 



t DEsr-ANimES, 'C. R.,' vol. 103, p. 375, 1886. 

 t HASSEMIEHG, ' Me"m. Acad. St. Petersburg,' 1885 (7), 32, No. 1. 

 LEWIS and KING, ' Astrophys. Journ.,' vol. 16, p. 162, 1902. 



I 2 



