250 



DK. W. GEOFFKEY DUFFIELD ON THE 



(l) The Influence of the Density of the Material in the Arc. In Table XII. the 

 measurements made during the present research upon the lines due to traces of iron 

 in the nickel poles are compared with the displacements of the same lines produced 

 in an arc between solid iron rods. 



At the top of each column will be found the pressure at which the examination was 

 made, and opposite each spectrum line the ratio of the displacement of the diluted to 

 that of the undiluted material. It will be seen that if the readings at 10 atmospheres 

 be left out of account (they are unduly high in nickel as we have already shown) there 

 is a smaller reading when the material is diluted. 



TABLE XII. 



The measurements of the displacements of the lines due to pure iron were made 

 some years ago both by the writer and by assistants, while those due to a small trace 

 of iron in the nickel poles were made Mr. PEARSE. In order to see if a comparison 

 between these readings was legitimate, Mr. PEARSE measured one of the original pure 

 iron arc photographs (at 70 atmospheres pressure), and his readings agreed so 

 excellently with the earlier ones that there was no hesitation in regarding the two sets 

 as strictly comparable. 



The ratio of the displacements with diluted to those with pure iron is, with the 

 exception noted above, less than unity, suggesting that the density of similar atoms 

 influences the displacement ; if the displacement depends upon the proximity of 

 similar vibrating centres it is to be expected that the displacements would be more 



