KFI'KCT OF PRESSURE UPON ARC SPECTRA. (JOI. I > 51 



The Effect of Pressure upon Arc Spectra. 

 No. 4. Gold. [PLATES 2-4.] 

 (Received May 6, Read June 9, 1910.) 



CONTENTS. 



Page 



1. Preliminary 51 



2. The spectrum of the gold arc at atmospheric pressure 52 



3. The behaviour of the gold arc under high pressures 54 



(1) Manipulation ' 54 



(2) Colour 55 



(3) Intensity 55 



(4) A change in a physical property of the gold 56 



(5) The occurrence of the calcium lines H and K 56 



4. The photographs 58 



(1) Region investigated 58 



(2) Description of the plates 59 



5. The broadening of the lines 60 



6. The displacement of the lines 62 



(1) The measurement of the photographs 62 



(2) Table of displacements 62 



(3) Displacement diagram 63 



(4) Relation between the displacement and wave-length 63 



(5) Resolution into groups of lines 69 



(6) Relation between the pressure and the displacement 71 



(7) Displacement and reversal of lines 72 



7. Changes in relative intensity under pressure 72 



Summary of result*. (See 'Roy. Soc. Proc.,' A, vol. 84, p. 121, 1910.) 



1. Preliminai-y. In the first part of this paper the apparatus and method of 

 taking the photographs under pressure have been described. The investigation of 

 the gold arc was carried out in precisely the same way, and the spectrum has now 

 been photographed under pressures ranging from 1 to +200 atmospheres. 



The photographs were taken in September, 1908, but the writer's absence in 

 Australia necessitated a delay in measuring them and in presenting the results. 



Though not exhibiting the remarkable features of the silver arc, the photographs 

 testify to the powerful means that an increase in the pressure of the surrounding 

 medium affords for displaying the different characters and properties of the individual 

 lines. Moreover, an investigation over a wider range of wave-lengths than the writer 

 has before attempted has brought nearer the solution of the problem of the relation 

 between the displacement and the wave-length, while the extension of the pressure 

 to +200 atmospheres enables us to investigate more thoroughly the relation between 

 the pressure and wave-length. 



H 2 



