PRESSURE EFFECT UPON ABSORPTION. 571 



The effect of pressure upon the ruby is similar to that of a lowering 

 of temperature in that the characteristic red doublet is sharpened and 

 shifted toward the violet by both agencies. An increase in pressure 

 did not, however, bring out the bands which are referred to by the 

 investigators mentioned as coming out at low temperatures. 



Summary. 



A comparison of the pressure effects with those due to other agencies 

 shows that, in general, the changes which take place in the absorption 

 spectra upon increase of pressure are similar to those which take place 

 upon lowering the temperature or decreasing the concentration. The 

 similarity of these three effects, especially in the case of neodymium 

 solutions, is very marked in that the same absorption bands are 

 affected in the same way, becoming narrower and sharper unsym- 

 metrically. There are, however, some differences in the effects due 

 to these different sources. There is a slight shift in the position of the 

 neodymium bands upon increase in pressure which does not take 

 place upon lowering of temperature or decrease of concentration. 

 This shift is, as has been mentioned, toward the red in most cases but 

 toward the violet in the case of band 4272. The only neodymium 

 solutions which show appreciable shift with rise in temperature are 

 those 23 to which some dehydrating agent such as calcium chloride 

 has been added. It is also observed that bands barely visible under 

 normal conditions are brought out more strongly by either increase of 

 pressure or lowering of temperature but this effect is not produced by 

 dilution. In order to bring more bands into prominence by change in 

 concentration, solutions of neodymium salts must be made more 

 concentrated rather than less as may be observed from Figure 3. 



It is not possible, from the observations made in this work, to make 

 any exact quantitative comparison of the effects upon the absorption 

 spectra due to different agencies but a study of the photographs of the 

 absorption spectra of neodymium ammonium nitrate taken at differ- 

 ent pressures, temperatures, and concentrations makes possible a 

 rough estimate of relations. The lowering to the temperature of 

 liquid air of a thin layer of concentrated solution of neodymium 

 nitrate giving an absorption spectrum at room temperature similar to 

 that observed in some of the solutions used for pressure work, pro- 

 duced a much greater effect in sharpening and intensifying the bands 



23 Jones and Strong, Am. Chem. Jour., Vol. 43, p. 130. 



