72 ABSORPTION SPECTRA OF SOLUTIONS. 



chemists of this company have always shown a spirit of cooperation with 

 scientific work on the rare elements that is very unusual, and for which 

 men of science, working in this field, owe them a lasting debt of gratitude. 

 The praseodymium used in this work was practically free from neo- 

 dymium, containing only a few hundredths of 1 per cent. From the 

 spectrograms it would appear that the neodymium used contained about 

 6 per cent of praseodymium. The erbium, of course, contained quite a 

 considerable amount of impurities. 



NEODYMIUM CHLORIDE IN WATER BEER'S LAW. (See Plates 59, 60, and 72 B.) 



Five different sets of solutions were made up, covering as wide a range 

 of concentrations as possible, the object being not only to test Beer's law 

 thoroughly, but also to get as complete a map as possible of the absorption 

 spectrum of neodymium chloride. In very concentrated solutions a cer- 

 tain group of bands may appear as a single band, due to the widening of 

 the individual bands or to general absorption in the region considered. By 

 diminishing the concentration such a "band" breaks up gradually into its 

 components, and hence, to map completely the absorption spectrum, it is 

 necessary to work over a wide range of concentrations. 



If the object were simply to "map the spectrum" this could, of course, 

 be most conveniently done by keeping the depth of layer constant and 

 changing the concentration through a sufficient range, thus getting the 

 complete spectrum on a single film; but since the chief object here was to 

 test Beer's law it was necessary to make several sets of solutions covering 

 different ranges of concentration. The concentrations of the solutions 

 used in making the negative for A, Plate 59, beginning with the one whose 

 spectrum is adjacent to the numbered scale, were 3.40, 3.02, 2.72, 2.38, 2.17, 

 1.90, and 1.70; the corresponding depths of cell being 12, 13.5, 15, 17, 19, 

 21.5, and 24 mm. For B, Plate 59, the concentrations were 3.40, 2.55, 

 1.70, 1.13, 0.80, 0.57, and 0.43; the corresponding depths of absorbing 

 layer being 3, 4, 6, 9, 13, 18, and 24 mm. For A, Plate 60, the concentra- 

 tions were 1.70, 1.27, 0.85, 0.57, 0.40, 0.28, and 0.22; for B, Plate 60, 

 they were 0.85, 0.63, 0.42, 0.28, 0.20, 0.14, and 0.11, and for B, Plate 72, 

 0.42, 0.31, 0.21, 0.14, 0.10, 0.07, and 0.055; the depths of absorbing layer 

 were in each case the same as in B, Plate 59. It will be noticed that begin- 

 ning with B, Plate 59, the concentrations used in each succeeding set are 

 just halved each time. 



The most concentrated solutions appeared brownish-yellow in their 

 bottles, from which the color changed on dilution to a yellowish-pink, the 

 color being extremely faint in the most dilute solutions. 



The exposures to the light of the Nernst lamp and spark were, respec- 

 tively, 1 and 2 minutes, the slit having a width of 0.01 cm. The exposures 

 and slit width were not varied in the work recorded in the present chapter, 

 the object being to make the spectrograms as nearly comparable as possible. 



Both A and B of Plate 59 show the presence of some general absorp- 

 tion in the ultra-violet, which decreases quite rapidly with dilution. The 

 absorption bands also narrow somewhat with decrease in concentration, 

 especially from 3.4 normal to about 1.7 normal. For concentrations less 



