EFFECT OF PRESSURE UPON ARC SPECTRA. -GOLD. 61 



The shape of the curve a suggests that it may be resolved into two curves due to two 

 different vibrating systems, of which one, 2 , may lie derived from the 

 other, by some effect of pressure such as loading or other suitable 

 means. Similarly for the other curves. 



Under higher pressures the principal part of the line (a,) becomes 

 limader and loses in intensity relatively to the background (which grows stronger) 

 into which it gradually merges, until at 100 atmospheres the intensity curve is like 

 that of the copper lines. The sharp appearance of the line is thus gradually lost and 

 the result is a broad, diffuse and soft line. 

 Examples of these lines are : 



(a) Symmetrical. 



ll, 4065-22 (Plate 3). This is the only very strong line that is symmetrically 

 broadened. It preserves its character from 1 to 200 atmospheres. 



(/>) Unsymmetrical unilateral. 



o2, 4315-45 1 The fact that at high pressures, e.g., 150 and 200 atmospheres 

 pi, 4437-44 I (Plate 4), these lines show some broadening on the violet side as 

 ql, 4488 '46 | well as the red makes it evident that this class of line cannot be 

 q2, 4607 "80 J rigorously separated from the following class. 



(c) Unsymmetrical bilateral. 



f'2, 3898-04 (Plate 2). 



r2, 4792-79. 



In this class the broadening is unequal on the two sides, and it is obviously 

 intermediate between (a) and (b). Line r2 is very broad, and extends over 90 A.U. 

 at 40 atmospheres, the greater extension being towards the red. The inequality in 

 the wings is not so marked as in f2, which approaches more closely to type a, whereas 

 r2 resembles lines of type b. 



4. The Unsymmetrical broadening may be greater towards the red or the violet end 

 of the spectrum. 



A few lines only have been observed to broaden out more towards the violet end of 

 the spectrum than the red, and these all occur below X = 4000. As a rule, the lines 

 are not strong, and become very diffuse under pressure (e.g., Plate 2, line el). These 

 are the lines that appear to be displaced towards the violet, but it is not yet definite 

 that the position of maximum intensity has been displaced in that sense ; the writer's 

 measurements of the line bl point to a displacement towards the violet at two 

 pressures, but he wishes to examine further instances of lines broadening to the violet 

 under pressure before pronouncing definitely upon it. 



5. The broadening is different for different linea 



Some lines remain comparatively fine when the pressure is increased, others are 

 very much broadened and diffuse, and a third class become faint hazy patches. The 



