12 ABSORPTION SPECTRA OF SOLUTIONS 



change with rise in temperature. While the percentage change in the 

 former of these two bands is perhaps greater, this shows very poorly on 

 the prints from the original films. The X4275 band is very sharp at the 

 lower temperatures, but shades rapidly towards the red as the temperature 

 is raised, the X5800 band, which is most affected by temperature changes, 

 showing decidedly more widening with the acetate than with the chloride. 

 This is exactly what we should expect if this band were associated with the 

 undissociated molecules of the salt in question. The acetate, being a salt of 

 a very weak acid, is dissociated considerably less than the chloride, and con- 

 sequently the change is greater in the case of the acetate where there are 

 present a larger number of molecules. 



The facts, then, are : The number of molecules in a given concentration of 

 neodymium acetate is greater than in the corresponding concentration of 

 neodymium chloride. Hydration decreases with rise in temperature. The 

 band X5800 is more marked in the acetate than in the chloride, and widen- 

 ing with rise in temperature indicates that it is in some way associated with 

 the hydrated molecules. 



NEODYMIUM SULPHATE IN WATER AND COBALT CHLORIDE IN WATER. 



(See Plate 7.) 



On account of the slight solubility of neodymium sulphate in water, only 

 the saturated solution was studied. The depth of cell was 10 cm. and the 

 temperatures, beginning with the strip nearest the numbered scale, were 20, 

 45, 75, 90, 115, and 140, respectively. 



It is seen that the first four strips of A (neodymium sulphate) show the 

 regular widening of X4275 and X5800. In strips 5 and 6 all of the bands 

 decrease in width. This is especially noticeable in bands X5800, X5100, and 

 X5225. This is no doubt due to the fact that some of the salt crystallized 

 out at this temperature, and the solution consequently became more dilute. 

 When the cell was opened it was found that nearly all of the salt had crys- 

 tallized out. 



It is, however, obvious that the sulphate presents no exception to the 

 general rule that the bands widen with rise in temperature. This is cer- 

 tainly true up to 115, at which temperature the crystals form rapidly, and 

 the effect of increase in dilution more than overcomes the counter effect of 

 rise in temperature. 



B is the spectrogram of a solution of cobalt chloride, 1 cm. deep and 0.25 

 normal. The temperatures, beginning with the strip nearest the numbered 

 scale, were 12, 32, 52, 76, 92, 112, 132, and 152. This plate shows an 

 intense absorption in the violet up to X3600; also a broad, hazy band with its 

 center near X5100. On account of the haziness of the cobalt bands, it is 

 difficult to discuss them in detail. The change produced by rise in temper- 

 ature, however, is very slight. The cobalt salt was hydrolyzed very greatly 

 at the higher temperatures, and this also interfered, with the study of its 

 absorption. 



