PRESSURE EFFECT UPON ABSORPTION. 567 



bands of solutions and of crystals of the rare earth salts become finer 

 and increase in intensity with lowering of temperature and are remark- 

 ably narrow at the temperature of liquid air. Becquerel states that 

 the width of the absorption bands in some substances varies propor- 

 tionally with the square root of the absolute temperature and that the 

 total amount of energy absorbed at low temperatures is greater than 

 the total amount absorbed at high temperatures. 



Marked changes are also produced in the absorption spectra of 

 neodymium solutions by changes in concentration as may be observed 

 from Figure 3. Bands 5800, 5200, and 4272, the ones most affected 

 by variations in pressure or temperature, are also most influenced by 

 changes in concentration. These bands are resolved and narrowed 

 unsymmetrically by dilution much as they are by either increasing the 

 pressure or lowering the temperature. The resemblance between 

 the effects of pressure and dilution upon band 4272 is especially marked 

 as may be observed by comparing the pressure effect described in this 

 paper with the effect produced by dilution as described by Jones 

 and Anderson. 14 



The changes produced in the neodymium absorption spectra by 

 increasing the pressure, lowering the temperatures or by dilution are 

 similar to those which have been observed by Nichols and Howes 15 in 

 the uranyl salts upon lowering the temperature to that of liquid air. 

 The absorption bands of these salts occur in groups which are poorly 

 resolved at ordinary temperatures. The component bands of the 

 groups are sharpened and intensified by reduction in temperature 

 and the envelope of the individual groups, as well as the envelope 

 of the groups taken as a whole, is pinched off more on the red side 

 than on the violet side. Some bands which appear on the red side of 

 the group at 20° are weakened or caused to disappear entirely by a 

 reduction of temperature to —180°. In general, the effect of cooling 

 is that the absorption spectrum, considered as a unit, suffers a narrow- 

 ing on cooling which is more marked toward the red than toward the 

 violet end. 



Praseodymium Salts. 



The absorption bands of praseodymium salts are wide and poorly 

 defined and the effect of pressure upon the solutions observed in this 

 work, the sulphate and the nitrate, was found to be very small. A 



14 Jones and Anderson, Carnegie Publication 110, p. 88. 



15 Nichols and Howes, Carnegie Publication 298, Ch. V, pp. 61, 79. 



