PRESSURE EFFECT UPON ABSORPTION. 563 



shown on the vertical axis. The figure shows nothing with regard to 

 relative intensities of bands but it gives a good idea of the absorption 

 bands present in different concentrations. For most of the high 

 pressure work, a .2 normal solution was found to be most satisfactory 

 which, with the thickness of about 1 .4 cm. of cell used in the apparatus, 

 gave an absorption spectrum comparable with that of an N/64 solu- 

 tion 20 cm. thick as shown in Figure 3. 



An increase of pressure was found to produce changes in the appear- 

 ance of the absorption spectra of neodymium solutions which were 

 observed both visually and photographically but which, on account 

 of the low dispersion of the spectrograph used, are not of sufficient 

 magnitude to reproduce satisfactorily. In general, the following 

 changes in the absorption spectra accompany an increase of pres- 

 sure: — 



1. — Some bands show a marked change with pressure increase while 

 others show little or no effect. The bands most affected are the 

 group between 5717 and 5865, the doublet 5205 and 5222, and the 

 sharp blue band 4272. (The first group of these bands will be referred 

 to hereafter as band 5800, and the doublet will be referred to as band 

 5200.) 



2.— The effect of increase in pressure is to narrow and intensify 

 the bands and to cause the component bands of a group to become 

 more distinctly resolved. The narrowing of the bands is usually 

 unsymmetrical, most of the reduction in width taking place on the 

 red side. 



3. — Some bands which are weak and diffuse at normal pressure are 

 brought out at high pressures. 



4. — An increase in pressure causes a slight shift in the position of 

 certain bands in the red end of the spectrum toward the violet and a 

 shift of the sharp blue band, 4272, toward the red. 



Neodymium Ammonium Nitrate. 



A very sharp absorption band at 7324, 6 not shown in Figure 3, 

 which came out only on plates dyed to make them red-sensitive, was 

 shifted toward the shorter wave-lengths upon application of 3500 

 atmospheres pressure. The width of this band is about 20 7 Angstroms 



6 The wave-length of this band is taken from Little's Text-book of Inor- 

 ganic Chemistry, p. 288. 



7 The values given for the width, narrowing, and shift of absorption bands 

 are only approximate since the poor definition of some of the bands and the 



