EFFECT OF PRESSURE UPON ARC SPECTRA. GOLD. 55 



steadily and silently, but as the poles gradually became hotter, turbulent convection 

 currents were produced in the vapour, the arc became noisier, and small explosions 

 on the poles gave rise to outbursts of metallic vapour which frequently blew out the 

 arc. 



With increase of pressure the arc became more troublesome, and considerable 

 difficulty was experienced in obtaining photographs at 5, 10, and 20 atmospheres, 

 though at higher pressures the arc again became steadier and more easy to 

 manipulate, behaving in this respect very differently from the copper arc. 



Throughout the whole range of pressure, from 1 to 200 atmospheres, the steadiness 

 with which the arc burned was found to depend upon two factors : (l) the coolness of 

 the poles and (2) the freshness of the air supply. It was found expedient during an 

 exposure, when the arc burned badly, to release a considerable quantity of the air 

 from the pressure cylinder and to admit a fresh supply, which both cooled the poles 

 and diluted the impure air containing the products formed by the arc burning in air. 

 This was always effectual in improving the steadiness and brightness of the arc. 



As in the case of the iron, copper, and silver arcs, with poles composed of the pure 

 metal, the exposure consisted of a number of short-lived arcs. A difficulty that was 

 accentuated in the manipulation of the gold arc was occasioned by the ease with 

 which the lower pole melted, causing the arc to be frequently formed behind the 

 upper pole in such a manner that its light was screened from the window. It 

 required some experience with the feed-wheels to cope with this trouble. 



(2) Colour. At low pressures the colour of the gold arc was not very determinate, 

 but is best described as mauve ; in comparison with the copper and silver arcs it 

 seemed soft and subdued, though it did not differ from them to any noticeable extent 

 in actinic power, requiring about the same exposure when it burned well. 



With increase of pressure it became whiter, and at 200 atmospheres was not very 

 different from the blue- white of the silver arc under the same pressure. 



(3) Intensity. The brightness of the gold arc increased with the pressure of the 

 surrounding air, but was never, as far as visual observations are reliable, as bright as 

 the copper or silver arcs at corresponding pressures. Attempts were made as before 

 to measure the intensities photometrically, but the fluctuations in the brightness 

 from instant to instant were too great to enable this to be done with consistent 

 results. 



The brightness is chiefly affected by those factors upon which the easy running of 

 the arc depends, namely, (1) the temperature of the poles, and (2) the freshness of 

 the air supply in the cylinder, which, besides influencing the intrinsic brightnew, 

 diminished by absorption the amount of light reaching the window of the pressure 

 cylinder. 



Other difficulties which were encountered lay in (3) the variable length of 

 as it jumped about on the irregular poles (which affected the current flowing through 

 it), and (4) the deposit of a fine material upon the window, which, though not serious 



