EFFECT OF PKESSURE UPON ARC SPECTRA. NICKEL. 251 



marked at high pressures, since increasing the pressure reduces the mean free path 

 between similar molecules just as increasing the total number of molecules does. 



The evidence thus favours the amount of the displacement being dependent in part, 

 at least, upon the amount of material present, a conclusion which is in keeping with 

 that arrived at from the consideration of the variation of the displacement with the 

 intensity of the line. 



The chief sources of contamination of the nickel poles are iron, cobalt, and 

 manganese. There is some doubt in one or two instances as to the origin of the lines ; 

 lines 136 and 137 for instance are very much alike under pressure, both in amount of 

 displacement and in their intensities, but one is ascribed to nickel and the other to 

 iron. 



The feature of the lines due to impurities is that they remain fairly compact even 

 at very high pressures, and do not spread out to the same extent or become so foggy 

 as the nickel lines. 



It is interesting to note that all the lines of the highest intensity due to iron do 

 not appear in the nickel spectrum. Some lines seem to characterize the spectrum due 

 to only a small quantity of material. For example, of the two iron lines (JJ?) 4143'50 

 (10) and 4143'96 (10), only one, the latter, appears in the nickel spectrum. Of the 

 two iron lines (150) at 4250'2 (10) and 425CT9 (10), both classed as of intensity 10, 

 only the latter appears, similarly, only the last-named of the two iron lines 4271 '30 

 (10), 4271'23 (10) is visible upon the plate. It is important to note that it is the line 

 which is self-reversed which most readily shows itself, it is the less refrangible line in 

 the pairs quoted. 



It has always been surprising to the writer that no dependence upon the density 

 of the material manifested itself in previous experiments. On one view one might 

 expect the general magnetic field of the surrounding atoms to influence the frequency 

 of any particular atom, and since, presumably, this general field depends upon the 

 nature of the atom the amount of material present or the nature of the surrounding 

 gas should have some effect, but nothing definite has hitherto been observed. On 

 another view, the specific inductive capacity of the medium in which the atom under 

 consideration is immersed should similarly affect the frequency, but this has teen 

 tested without positive result. 



The explanation seems to be that in the arc the isolation of a molecule is never 

 actually accomplished, that the vaporization of the metal, even if only a trace of it 

 be present, involves the liberation of an immense number of atoms of that element all 

 in close proximity, so that this incandescent mass behaves very much as though it 

 were isolated from the other materials in the arc, rendering it very difficult to influence 

 the immediate environment of an atom, since only the few atoms on the outskirts 

 of this mass are affected by the inductive properties of the surrounding gas (or 

 other metallic vapour produced by the vaporization of the poles), consequently 

 the predominant frequency is that of the atom surrounded by similar atoms. 



