94 ABSORPTION SPECTRA OF SOLUTIONS. 



ably from solution to solution, even when the percentage of water is much 

 greater; for example, changes may be noticed in passing from the solution 

 containing 100 per cent water to the one containing 50 per cent. This is 

 undoubtedly due to the change in the dissociation of the dissolved salt, 

 which, in the case of the nitrate, modifies the spectrum; while in the 

 case of the chloride no such change was noted, except at the very greatest 

 concentrations. The spectrogram, therefore, shows a superposition of the 

 two effects, and if this is borne in mind everything about it is perfectly 

 clear without further discussion. 



NEODYMIUM NITRATE IN MIXTURES OF ACETONE AND WATER. (See Plate 67 B.) 



The concentration of the neodymium nitrate was constant throughout 

 and equal to 0.6 normal. The percentages of water in the solutions, begin- 

 ning with the one whose spectrum is adjacent to the numbered scale, were 

 0, 2.6, 5.3, 8, 10.6, 13.3, and 16. The common depth of absorbing layer for 

 all the solutions was 0.5 cm. 



In this, as well as in the case treated under Plate 76 A, we have to do 

 with a superposition of two effects. First, the change produced in the 

 water solution resulting from decreased dissociation with the addition of 

 the non-aqueous solvent; and secondly, the change in the structure of 

 the bands which takes place when the amount of water has been decreased 

 so far that the molecules of the dissolved substance are no longer able to 

 be surrounded by the usual number of water-molecules, but become sur- 

 rounded by molecules of the non-aqueous solvent in the present case 

 acetone. In the solution whose spectrum is nearest the narrow, comparison 

 spark spectrum, the percentage of water being only 16 per cent, the dis- 

 sociation is already rather slight, so that the spectrum is approximately 

 that which we would observe in a very concentrated aqueous solution of 

 the salt in a layer only about a millimeter in depth. With decrease in the 

 amount of water the change is easiest to follow in the more intense of the 

 bands in the green, this being the one which differs most in the acetone 

 and concentrated aqueous solutions. It will be noticed that the most 

 marked change in this band takes place in passing from the fifth to the 

 third strips, counting from the scale; that is, when the water content of 

 the solvent changes from 10.6 to 5.3 per cent. This agrees substantially 

 with what we found to hold in the case of solutions of the chloride in mix- 

 tures of water and the alcohols. 



PRASEODYMIUM CHLORIDE IN WATER BEER'S LAW. (See Plate 77.) 



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 2.56, 1.92, 1.28, 0.85, 0.60, 0.42, and 0.32. For B the concen- 

 trations were 0.85, 0.63, 0.42, 0.28, 0.20, 0.14, and 0.11, the depths of 

 absorbing layer being respectively 3, 4, 6, 9, 13, 18, and 24 mm. 



The solutions of praseodymium chloride are all green or yellowish-green, 

 only the intensity of the color changing with change in the concentration. 



For these solutions Beer's law holds very exactly, excepting for the 

 extreme ultra-violet absorption in A, and the yellow bands in the two or 

 three most concentrated solutions of A. 



