252 DK. W. GEOFFREY DUFFIELD ON ARC SPECTRA. NICKEL. 



Decreasing the amount of impurity present would make the outer portions of its 

 incandescent vapour relatively more important, and increase of pressure would achieve 

 the same end by decreasing the average distance of each vibrating centre from the 

 molecules of the surrounding gas which might then exert an influence which is 

 negligible when the pressure is low. We thus expect to find a reduced displacement 

 when the material is greatly diluted and a relative reduction with increased pressure. 

 The former is supported, and the latter suggested, by Table XII. We have assumed 

 that the surrounding atmosphere is less effective in producing displacement than an 

 atmosphere of similar molecules, but it should not be impossible for the opposite to be 

 the case, according to the nature of the substances employed, unless it can be proved 

 that similar molecules alone have influence upon a radiating molecule. 



Approaching the problem of the structure of the arc from a different direction, the 

 writer has been impressed by the importance of the part played by the surrounding 

 gas in maintaining the arc ; it would seem to require that each metallic atom is, for a 

 brief interval at least, associated with one of the atoms of the surrounding gas so 

 that something akin to chemical action takes place between them (or at least 

 involves the influence of what may be called chemical affinity). If this is the case it 

 would appear contrary to the view of the density effect just put forward, since that 

 does not contemplate the commingling of the individual atoms of the metal with those 

 of the surrounding gas. But it is further possible that the spectrum line is due, not 

 to single atoms but to a system, such as one consisting of a metallic atom combined 

 or interacting with one atom of the surrounding gas (the function of the latter being, 

 by its interactions with the atom, to excite it to emit its characteristic radiation), 

 and that such systems form the aggregate already alluded to, and that the frequency 

 of the resultant spectrum line is characteristic of this particular atom system in its 

 environment of similar systems. These systems would not be interfered with by 

 foreign systems until either the reduction of the amount of material below a certain 

 minimum amount or the increase of pressure made the proximity of foreign atomic 

 systems relatively more important. 



Possibly this consideration is responsible for the decrease in the rate of increase of 

 displacement with pressure when the latter is high. 



The writer is indebted to Dr. SCHUSTER, at whose suggestion the series of experi- 

 ments upon the effect of pressure upon spectra were begun some years ago, and to Sir 

 ERNEST RUTHERFORD in whose laboratory the photographs were taken, and expresses 

 his thanks to them for having placed the necessary apparatus at his disposal. 



The photographs were measured by Mr. PEARSE in a careful and thorough manner. 

 Part of the expense of this research was defrayed by a grant from the Government 

 Grant Committee. 



