EFFECT OF DILUTION ON ABSORPTION OF LIGHT. 23 



band X5750 narrows about 40 a.u. from the third to the second strips, and 

 about 15 a.u. with the next dilution. 



The concentrations used in B were again just half those in A, the most 

 concentrated solution being one-half saturated, with succeeding dilutions of 

 100 and 5 times, respectively; the depths of cell, beginning with the strip 

 farthest removed from the scale, were 0.5 cm., 50 cm. and 250 cm. 



Again, only bands X5220 and X5750 are changed, but with the acetate the 

 change extends farther with the more dilute solutions. In a word, the nar- 

 rowing of the bands with dilution is more marked in B and C than in the case 

 of the chloride, bromide, and nitrate. The group of bands near X3500 is not 

 altered with dilution. 



The concentrations used in C are again half of those in B, and the corre- 

 sponding depths of cell the same as used throughout this plate. Their respec- 

 tive sequence is the same. In this spectrogram only band X5750 changes, 

 and, indeed, this is the only salt of neodymium with which a change with 

 dilution has been noted in so dilute a solution. This band narrows about 20 a.u. 



Neodymium acetate, being the salt of a weak acid, is of course less dis- 

 sociated at any given concentration than a salt of a strong acid. This salt 

 approaches complete dissociation much more slowly than the others studied. 

 Spectrograms A, B, and C show also that the changes caused by dilution are 

 more marked and extend into more dilute solutions than with the chloride, 

 bromide, or nitrate. In a word, the changes in the absorption bands due to 

 dilution seem to follow the change in dissociation; that is, they are a direct 

 function of the number of molecules present. 



PRASEODYMIUM CHLORIDE IN WATER. (See Plate 18.) 



The concentrations of solutions used in making A, beginning with the 

 strip farthest removed from the numbered scale, were 2.56, 0.0256, and 

 0.00512 normal, the corresponding depths of absorbing layer being 0.5 cm., 

 50 cm., and 250 cm., respectively. 



The absorption is complete in the ultra-violet up to about X3100. The 

 bands of praseodymium are for the most part broad and have well-defined, 

 sharp edges. The violet edge of band X4450 is very sharp and unchanged 

 by dilution, while the hazy red edge is hardly affected. Band X4675 narrows 

 towards the violet about 20 a.u., while band X4830 is entirely unchanged. 

 The broad band X5900, with slightly hazy edges, shows a total narrowing of 

 about 25 a.u. 



The concentrations of B and C, beginning with the strips farthest from 

 the numbered scale, are 1.28, 0.0128, and 0.00256 normal and 0.64, 0.0064, 

 and 0.00128 normal, respectively. The corresponding depths of absorbing 

 layer were the same as in A. None of the bands is affected by dilution, 

 and we may say that Beer's law holds very well for praseodymium chloride, 

 except for bands X4675 and X5900 in the most concentrated solutions. Even 

 here the change is very slight and not to be compared with corresponding 

 changes with dilution in salts of neodymium. 



