EFFECT OF PRESSURE UPON ARC SPECTRA. SILVER. 35 



The ease with which the arc burned at any pressure seemed dependent upon the 

 amount of air present ; when the cylinder was freshly filled the arc burned steadily 

 for some time, though not often for longer than one minute, without requiring the 

 poles to be brought together, but later on the arc required more attention and 

 frequent re-striking ; if, however, the gases were allowed to escape from the cylinder 

 and fresh air introduced, steadiness was again obtained. A plentiful supply of air 

 thus appears to be necessary for steady running, but the cooling of the poles is also 

 a factor that makes for success. Not only did the steadiness of the arc decrease at 

 any particular pressure as time advanced, but also the brightness, the brilliance of 

 the image on the jaws of th'e slit gradually waning as the arc burned, until a fresh 

 supply of air replaced the old. 



And that the readiness with which air has access to the poles is connected with 

 the brightness is further borne out by the increased brightness as the pressure is 

 increased, provided that arcs newly supplied with air are compared ; for instance, 

 the arc under a pressure of 200 atmospheres, when first struck, is very much brighter 

 than the arc under a pressure of 50 atmospheres when first struck. This indicates 

 also that the temperature of the arc under high pressure is very much greater than it 

 is at normal atmospheric pressure. Photometric measurements of the intensity of the 

 light emitted under different pressures have been attempted, but the intermittent 

 nature of the arc has not permitted accurate determinations. Distinct changes were 

 observed in the colour of the arc as the pressure was increased ; at low pressures it 

 maintained its characteristic greenish appearance, but at the highest pressure reached, 

 200 atmospheres, it was, when the air was fresh, as white as a cart>on arc ; at lower 

 pressures, or when the arc had been burning for some time, the greenish colour 

 returned, and my assistant pointed out to me two distinct tints besides the pure 

 white, namely, yellowish-green and green ; sometimes as the arc flickered about the 

 poles in an irregular manner these would appear in rapid succession, possibly dependent 

 on the varying length of the arc gap as it moved from point to point on the 

 electrodes. 



The silver contained a trace of lead, which is characterised by the line at 4058 "04, 

 and on one or two of the photographs at atmospheric pressure the cyanogen band at 

 3883 is visible and is due to the silver having been melted in a carbon reducing 

 atmosphere. As is usually the case when metallic arcs are burned in air under 

 pressure, some nitric acid was formed within the cylinder, and the air issuing from it 

 gave the characteristic smell of nitrogen peroxide. The appearance of some finely 

 divided particles of iron in the window-tube after the arc had been run for some time 

 at high pressures needs explanation, but it is considered to have either been introduced 

 with the air, which was stored in an iron cylinder, or to have scaled from the steel 

 rods which carried the silver electrodes, though this must have been greatly obviated 

 by the discs of asbestos which were strung on the silver poles and were a good fit in 

 the cylinder. 



F 2 



