EFFECT OF PRESSURE UPON ARC SPECTRA. SILVER. 43 



experiments is the replacement of the line spectrum of silver by a banded spectrum 

 when the pressure is large. A necessary condition for the production of this new 

 spectrum is that the arc burns steadily, because, as has just been pointed out, at the 

 moments of extinction some of the lines have been observed to flash out instan- 

 taneously, even at 100 atmospheres. On the negatives the bands are not always 

 easily distinguishable, but greater contrast has been obtained by using a slow Velox 

 paper, which enables the detail to be more carefully studied. 



(1) The Structure of the Banded Spectrum. The photographs show a number 01 

 bright isolated patches or bands, separated by dark intervals. The bright bands are 

 symmetrical about their centres, and are generally very broad, fuzzy and ill-defined, 

 but a few are as narrow as 2 or 3 A. U. 



There is no easily recognisable regularity in the spacings of these patches of light, 

 or bands, and their intensities differ considerably and show no dependence upon their 

 widths. 



The resemblance of this spectrum to an absorption spectrum is discussed under a 

 separate heading, p. 48. If the bands be observed visually they present a novel 

 appearance. In place of the steady spectrum which is observed with other metals, 

 there is a constant flickering of the spectrum and waves seem to pass across it, and 

 the bands appear more like tongues of coloured flames which are violently disturbed 

 by a breeze. Horizontal striae crossing the spectrum were also frequently noticed. 

 The whole phenomenon is thought to be due to violent convection currents in the 

 vapour surrounding the arc, which, being of varying density, cause continually 

 changing amounts of refraction of the rays which pass through it on their way to the 

 window. The constant play of shadows over the inner surface of the window, which 

 is easily visible because of the fine deposit upon it, indicates that some such action as 

 this is in progress, and the striae may be due to shadows falling upon the slit. 



The Behaviour of the Banded Spectrum under Pressure. The bands first appear 

 on the photographs at 20 atmospheres, where many are resolvable into a number of 

 very fine lines, but, as it was thought possible that longer exposure at a lower 

 pressure might show a trace of them, the normal silver arc was photographed with an 

 exposure ten times longer than usual, and with a very wide slit ( mm.) ; but there 

 was no sign of bands in this experiment though the slit included both poles and the 

 centre of the arc. 



(2) Broadening. As the pressure increases the bands strengthen and become a 

 more definite phenomenon; many grow broader (cf. 45697, which increases from 

 8 A. U. at 25 atmospheres to 13 A.U. at 80 atmospheres and 26 A.U. at 100 atmo- 

 spheres), and several, which are separately distinguishable at 25 atmospheres, become 

 merged into one another at higher pressures. 



At 100 atmospheres the bands have become so broad that they almost constitute 

 a continuous spectrum, though the characteristics of the banded spectrum are not 

 entirely lost. 



o 2 



