1898.] the Absorption Bands of Didymium Salts, etc. 41 



and general character in both salts, and the same intensities so 

 long as the solutions are equivalent and moderately dilute, yet 

 more concentrated equivalent solutions shew unequal variations 

 in the intensities of the absorptions, some being stronger with the 

 nitrate and others stronger with the chloride. 



The mixture of earths used contained praseodymia and neo- 

 dymia, samaria and some other earths, but was spectroscopically 

 free from lanthana, though not from yttria. Some of the oxide 

 was gently ignited, and equal quantities weighed out for solution 

 in different acids so as to give solutions of equivalent strengths 

 when diluted to equal volumes. The sulphate is so much less 

 soluble than the nitrate and chloride, that equivalent solutions of 

 these three salts could only be prepared in a rather dilute condition. 

 In the case of the nitrate and chloride the solutions were evapo- 

 rated to drive off all excess of acid and then redissolved in water. 

 They were interposed between a lime-light and the slit of the 

 spectroscope in tubes closed with quartz lenses at their ends so as 

 to concentrate the transmitted light on the slit. The spectroscope 

 had quartz objectives and calcite prisms. Also the spectra to be 

 compared were always photographed in succession on the same 

 plate, so that inequalities in the development, as well as in the 

 intensity of the light used, might be as far as possible avoided. 



Equivalent solutions of the sulphate, nitrate, and chloride gave 

 absorptions which were indistinguishable from each other. This 

 is in accordance with the theory which has been worked out by 

 Ostwald, that dilute solutions of salts which have one, and the 

 same, coloured ion, with various colourless ions, all shew the same 

 absorptions. 



The absorptions by a thickness of 6 inches of each of a series 

 of equivalent solutions of the chloride and nitrate were then 

 photographed, the strength of the solutions being regularly grad- 

 uated ; and subsequently the absorptions by a thickness of 3 inches 

 of the same series. The range of the spectrum photographed was 

 from about a wave-length X 522 to X 365, and again, on a second 

 set of plates, from about X 369 to X 315, i.e. nearly up to 8 of the 

 solar spectrum. 



The strongest solutions employed contained 21*6315 grms. 

 of the oxide which, after conversion into nitrate, or chloride, 

 was dissolved in 70 c.c. of water. The more dilute were prepared 

 from this by successive dilutions to |, £, %,... to ^ of the original 

 strength. 



Comparing the effects of the same thickness of solutions of the 

 various degrees of dilution, we find in the region between X 522 

 and X 365 : 



1. That all the bands produced by the dilute solutions are 

 strengthened, in both chloride and nitrate, in the stronger solutions, 



