80 ABSORPTION SPECTliA OF SOLUTIONS. 



perature, but from 20 to 160 it changes less than 25 a.u. There is no 

 appreciable change with temperature of the bands whose centers are near 

 X4425, X4650, and X4820. 



The above-described temperature changes take place only in concentrated 

 solutions. In very concentrated solutions all of the praseodymium bands 

 show a slight widening with rise in temperature. We shall see that increase 

 in dilution affects the bands of praseodymium salts only when the solutions 

 are fairly concentrated. Thus, rise in temperature and increase in concen- 

 tration produce the same effect on the absorption spectra of solutions of 

 praseodymium nitrate. 



The effect of rise in temperature on the absorption spectra of solutions of 

 uranyl nitrate is a general widening of the bands, with a slight shift of the 

 center towards the red. The general absorption ending near X3500 moves 

 rapidly towards the red with rise in temperature. All of the eleven bands 

 between X3500 and X4600 become more diffuse and broader with rise in 

 temperature, the X4180 band being most affected. The red edge of this 

 band shifts as much as 25 a.u. from 40 to 120. 



The uranyl sulphate bands X4175 and X4325 have their centers shifted 

 towards the red about 25 a.u. for a temperature range of from 20 to 185. 

 The band X4750 remains unchanged, while the red edges of X4325 and X4550 

 shade rapidly towards the red. All bands below X4500 become very diffuse 

 as the temperature is raised, and at the highest temperatures are a single, 

 broad, hazy absorption band extending from X4000 to X4400. 



The most marked widening is in the uranyl sulphate bands X4100, X4200, 

 and X4350, the center of each of these bands being slightly shifted towards 

 the red. The broad, hazy bands X5100, X5600, and X6200 are not appreci- 

 ably affected by changes in temperature. 



None of the uranyl acetate bands seems to undergo change with dilution; 

 all of the nine bands on the plate undergo change with rise in temperature, 

 becoming more diffuse. 



While some of the absorption bands of solutions are practically unaffected 

 by temperature, many of them widen as the temperature is raised. The 

 effect of rise in temperature is not to produce a symmetrical widening of 

 the bands, but most of the widening is towards the red. The violet edge of 

 the band usually remains pretty sharp. The red edge widens out, becoming 

 more hazy and diffuse. 



The effect of dilution on the absorption of light by solutions was early 

 studied by Ostwald and others, especially in connection with the theory of 

 electrolytic dissociation. It was known that both molecules and ions in 

 solution absorb light, and the question was whether they have the same or 

 different absorption. It was not possible to answer this question satis- 

 factorily by means of the prism spectroscope. It has been possible to solve 

 this problem by means of the grating. 



Jones and Anderson had shown that if molecules and ions absorb differ- 

 ently, the difference is slight. We therefore worked over a wide range in 



