EFFECT OF PEESSUEE UPON AEC SPECTEA. 



207 



necessary time of exposure, because under pressure the lines broaden and the energy 

 of vibration is spread over a greater area. 



Though the characteristic green appearance of the copper arc was observed at all 

 pressures up to 100 atmospheres, the red flame which frequently issues from the 

 normal arc appeared with great brilliance at 80 and 100 atmospheres.* 



At the conclusion of the experiments the copper rods were removed ; the lower was 

 found to be coated with black oxide, and the upper with a fine grey deposit which is 

 thought to be a basic nitrate. On the tips of the poles minute metallic globules had 

 been formed ; these when first examined lacked the red lustre characteristic of copper 

 and were of a silvery whiteness, but the red lustre appeared after some months. 



4. THE PHOTOGRAPHS. 

 (1) Method of Exposure. 



As in the previous work with the iron arc, the comparison spectrum under at- 

 mospheric pressure was photographed in the central strip of a plate with the spectrum 

 under pressure above arid below it. To ensure that no accidental displacements were 

 produced, the comparison spectrum was photographed both before ;ind after the one 

 under pressure. The arc was operated by the writer and the mirrors by an assistant. 



The following photographs have been obtained : 



Plates : Imperial Flashlight. Developer : Imperial Pyro-Metol Standard. Expo- 

 sure varied from 3 minutes at 5 atmospheres to 60 minutes at 100 atmospheres. 



(2) Description of the Plates. 



Plates 10 and 11 illustrate the behaviour of the copper arc under different pressures ; 

 Plate 10 includes the region X = 4050 to X = 4300, and Plate 1 1 the region X = 4350 to 

 X = 4600. The photographs, which are full-size positive reproductions of the originals, 

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

 bottom at 80 atmospheres, the central strip being always at normal atmospheric 

 pressure. 



* [Note added October 23, 1908. Subsequent observations up to 200 atmospheres show that the arc 

 becomes decidedly bluer at about 125 atmospheres, and nearly as white as a carbon arc at 200 atmospheres.} 



