96 ABSORPTION SPECTRA OF SOLUTIONS. 



one described in detail in discussing the X 4760 band for neodymium 

 chloride in mixtures of alcohol and water. 



The positions of the bands in the solution containing 16 per cent of 

 water are as follows : X 4390 to X 4470, X 4660 to X 4700, X 4800 to X 4825. 

 In the solution in pure alcohol they are X 4410 to X 4480, X 4690 to X 4715, 

 X 4810 to X 4840. Hence, it appears that the two most refrangible bands 

 have a slightly greater width in the water solution, while the X 4815 band 

 is more intense in the alcoholic solutions. 



The bands in the yellow show very well indeed the fact that here as 

 in the spectrum of neodymium chloride we have the coexistence of two 

 sets of bands when the water content of a 0.5 normal solution is in the 

 neighborhood of 8 per cent. The band in the yellow has already been 

 described under Beer's law, but as the concentration and depth of layer 

 are different here, the following will serve to indicate what the spectrum 

 of the 16 per cent water solution shows. Absorption begins at X 5850 and 

 rises to a maximum at about X 5900, then decreases to a minimum at X 

 5950, from which it again rises to a maximum at about X 5980, falling off 

 to zero at X 6000. The solution in pure alcohol shows the following : Weak 

 absorption begins at X 5800 and continues without material change up to 

 X 5880, where it falls almost to nothing. At X 5900 it begins to increase 

 and reaches a strong maximum at X 5955, falling off gradually to zero at 

 X 6000. The intermediate solutions show the gradual disappearance of 

 the bands characteristic of the aqueous solution, and the increase in inten- 

 sity of those belonging to the alcoholic solution, as the percentage of water 

 is gradually decreased. The maximum change takes place from the fifth 

 to the third strips, counting from the numbered scale, indicating here, as 

 with neodymium chloride, that the two sets have about half their normal 

 intensity when the water content of the solution is about 8 per cent, or 

 when the solution contains about 10 molecules of water per molecule of 

 the dissolved substance. 



PRASEODYMIUM NITRATE IN WATER BEER'S LAW. (See Plate 79.) 



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 3.2, 2.4, 1.6, 1.1, 0.75, 0.53, and 0.41. For B the concentrations 

 were 1.1, 0.80, 0.55, 0.33, 0.26, 0.18, and 0.14; the depths of absorbing 

 layer in both cases were 3, 4, 6, 9, 13, 18, and 24 mm. 



There is a great deal of absorption in the ultra-violet, the spectrum of 

 the most concentrated solution ending at X 4000, while that of the four 

 most dilute solutions of set A ends at about X 3650. The spectra shown 

 in B all end at X 3570. This absorption is not to be ascribed to the NO 3 

 radical, as its band lies beyond X 3300 in all the solutions thus far studied. 



The absorption bands do not differ materially from those of the chlo- 

 ride, except that they are a trifle more intense, due, no doubt, to the slightly 

 greater concentration of the nitrate solutions. Also, the violet and blue 

 bands show a slight deviation from Beer's law in the two or three most 

 concentrated solutions of A. 



