210 Mr Purvis, On the influence of great dilution on the 



bands at A418, A415, and A406 are faintly visible in both solu- 

 tions and slightly wider in the strong solution, whilst the broad 

 band at A. 403 seems to be wider also. 



The general effect appears to be that all the bands observed 

 are more diffuse in the concentrated solution of Didymium 

 nitrate than in the diluted solutions of equivalent thickness. 

 On comparing the strong solution of Didymium chloride with 

 that of the nitrate containing equivalent quantities of the salt it 

 is noticed that the absorption bands resemble those of the dilute 

 solution of the nitrate. There is no apparent difference between 

 the strong solution of Didymium chloride and the diluted 

 solution of Didymium nitrate. The only point of difference 

 seems to be that there is an extension of the general absorption 

 of the more refrangible end of the spectrum in the concentrated 

 solution of Didymium chloride, and this is comparable with that 

 in the strong solution of Didymium nitrate. If the concentrated 

 and the diluted solutions of Didymium chloride are compared it 

 will be noticed that there is no apparent difference in the various 

 absorption bands, and the only difference is again that of a more 

 general absorption of the more refrangible end by the concen- 

 trated solution. That is to say, in these experiments the 

 concentrated solution of Didymium chloride, the dilute solution 

 of Didymium chloride, and the dilute solution of Didymium 

 nitrate are similar as regards their absorption spectra, and the 

 only apparent difference is that mentioned above. 



If the concentrated and diluted solutions of the nitrate of 

 Erbium are similarly compared, it is noticed that there is the 

 same general differences as in those of- the Didymium nitrate. 

 The bands in the concentrated solutions are much more diffuse 

 than in the diluted solutions, whilst some of the very faint bands 

 disappear and the more sharply defined bands appear wider. 

 The band at about A, 539 almost disappears. The two bands at 

 about A, 536 of almost equal intensity appear to be more diffuse, 

 and the group of A 523 and A 527 merge into each other to form 

 a single broad band. The groups A, 491, A 488, and A, 480 are more 

 diffuse, and the bands A449 and A454 are almost completely 

 diffused into each other, as is also the case with A 404 and A407. 

 The bands at A418 and A422 disappear completely in the con- 

 centrated solution, whilst the band at A 441 does not seem to 

 suffer so marked a change. Again here, as in the case of the 

 salts of Didymium, there is an extension of the general absorption 

 of the more refrangible end in the concentrated solution. 



Also, if the concentrated and diluted solutions of the chloride 

 of Erbium are compared it is seen that there is no apparent 

 difference in the various bands, and that these strictly correspond 

 to trrose of the diluted solution of the nitrate of Erbium ; the 

 only marked difference being the greater extension of the general 



