76 



ABSORPTION SPECTRA OF SOLUTIONS. 



be understood that this table is not meant to represent what could be 

 seen or photographed in any one solution of neodymium chloride in water. 

 It merely records the position of all the bands which can be seen in a layer 

 from 3 to 12 mm. deep, when the concentration is varied from to 3.4 normal. 



2900 

 2985 

 3225 

 3390 

 3465 

 3505 

 H.vtO 

 3560 

 4180 

 4275 

 4290 

 4330 

 4615 

 4H45 

 4695 

 4760 

 4825 

 5090 

 5125 



Character. 



About 20 A. U. wide. 



About 25 A.U. wide. 



Narrow and sharp. 



Narrow, faint. 



Very intense, narrow. 



Rather wide. 



Very intense, narrow. 



Faint, narrow. 



Faint, hazy. 



Very intense and sharp. 



Very narrow, faint. 



Hazy edges. 



Rather wide and hazy. 



Very narrow, faint. 



Narrow, intense. 



Hazy edges, fairly narrow. 



Narrow and fairly intense. 



Narrow, intense. 



Rather wide and hazy. 



51MI5 



5222 



5255 



5315 



5725 



5745 



5765 



5795 



5830 



6235 



6260 



6270-6310 

 6360-6390 



6730 



6800 



6890 



7325 



7390 



Character. 



Very intense, narrow. 



Very intense, narrow. 



Narrow, intense. 



Hazy edges, faint. 



Narrow, intense. 



Very intense. 



Very intense. 



Intense, moderately narrow. 



Very faint and hazy. 



Fairly narrow. 



Very narrow, faint. 



Faint, hazy edges. 



Faint, hazy edges. 



Faint band. 



Moderately intense, hazy edges. 



Hazy edges. 



Very intense and narrow. 



Rather wide band. 



NEODYMIUM CHLORIDE IN WATER MOLECULES CONSTANT. (See Plate 61.) 



The dissociation of neodymium salts not having been determined, it 

 was assumed that their dissociation was the same as those of aluminium. 

 Although this may not be exactly true, yet the rate of change of dissocia- 

 tion with concentration will perhaps be practically the same for the two 

 metals, and that is the only thing which comes into account here. 



The concentrations of the solutions used in making the negative for A, 

 beginning with the one whose spectrum is adjacent to the numbered scale, 

 were 3.4, 2.7, 1.95, 1.44, 1.10, 0.86, and 0.69; the corresponding depths of 

 absorbing layer were 3, 4, 6, 9, 13, 18, and 24 mm. For B the concentra- 

 tions were 1.36, 1.10, 0.80, 0.59, 0.44, 0.35, and 0.28; the depths of the 

 absorbing layers were the same as in A. 



Since Beer's law holds so very accurately for neodymium chloride in 

 water, excepting at the very greatest concentrations, it is to be expected 

 that when molecules are kept constant all the bands would show consider- 

 able widening with dilution, and this is found to be the case. It will be 

 recalled, however, that the shading on the red side of the yellow band 

 showed considerable deviations from Beer's law, even at moderate dilu- 

 tions; and it was to see whether there is any connection between this 

 shading and the undissociated molecules that the present spectrogram was 

 made. Here it will be seen that the shading decreases when the concentra- 

 tion is changed from 3.4 to 1.95, then remains sensibly constant until the 

 concentration becomes as small as about 1.0, when it increases with further 

 dilution. It seems evident, then, that this shading can not be ascribed to 

 the undissociated molecules, any more than can the rest of the absorption 

 phenomena shown by these solutions. Apparently the absorption depends 

 only upon the number of neodymium atoms present, and is independent, 

 or nearly so, of whether these exist as ions or combined with chlorine to 

 form the chloride molecules. 



