572 avick. 



than that which was observed in the highest pressures used in these 

 experiments. The effect of a pressure of 3500 atmospheres was about 

 equal to the effect due to the lowering of temperature which took place 

 when the cooled specimen was supported in a Dewar flask above 

 liquid air and attached to a metal strip, the lower end of which dipped 

 into the liquid air. The temperature of this specimen was not meas- 

 ured but a rough estimate indicates that it was at least GO below zero 

 centigrade. A comparison of the pressure and concentration effects 

 shows that an increase of 1700 atmospheres pressure upon a .2 normal 

 solution of a neodymium nitrate is approximately equivalent to a 

 reduction of the concentration of this solution to one half its value, 

 that is, to making the concentration of the solution .1 normal. 



The changes observed in the absorption spectra upon increase of 

 pressure cannot be explained by any actual temperature or concentra- 

 tion change which might take place as a result of pressure since any 

 direct effect due to either of these sources is known to be usually in the 

 opposite direction. Upon application of pressure, the specimen be- 

 comes warmer but the heating effect amounts to only a few degrees 

 with the highest pressures used and this slight rise in temperature 

 rapidly disappears in the heavy steel cylinder. If correction were to 

 be made for it, the corrected value of the pressure effect would be 

 larger than the observed effect. Upon application of the highest 

 pressures used, the actual volume of the aqueous solutions was de- 

 creased about 10 per cent. If this decrease of volume under pressure 

 is equivalent to an increase in concentration, the effect of it should be 

 to broaden rather than narrow the bands, and correction for it should 

 make the actual pressure effect larger than the observed effect. 



The fact that such agencies as variations in pressure, temperature, 

 and concentration affect some of the absorption bands of solutions but 

 not others and the fact that the bands affected undergo changes which 

 are not the same for different bands, seem to indicate that there must 

 be more than one simple type of mechanism involved in the produc- 

 tion of these absorption bands. 



From a comparison of the absorption bands of neodymium crystals 

 with those of the same salts in solution, it is observed that the 

 bands in the crystals are sharper than those in the solutions under 

 normal conditions. Since absorption bands in solutions are made 

 sharper by pressure, it seems possible that solutions under pressure 

 may have something approaching structure tending to make the bands 

 more as they are in the solid state. However, until more is known 

 about the mechanism of the absorption of light, an explanation of the 



