PRESSURE EFFECT UPON ABSORPTION. 569 



the width of this absorption band varying with the concentration and 

 thickness of the solution used. Some of the photographs of the 

 spectra of these solutions showed that, upon increase of pressure, 

 the transmission in the red end of the spectrum was increased and that 

 in the blue end was decreased causing the color of the transmitted 

 light to become more red at higher pressures as it does at lower tem- 

 peratures, the greatest change taking place in concentrated solutions. 

 The wide, diffuse absorption band having its center at about 5200 was 

 observed to change its limits with increase of pressure but no quanti- 

 tative measurements could be made on account of difficulties already 

 mentioned in connection with the apparatus. 



Glass. 



Samples of glass made at the Corning Glass Works, some of which 

 were known to change color with variation in temperature, were 

 obtained through the kindness of Dr. H. P. Gage. The effect of 

 pressure upon these specimens was such as to produce a change in 

 color in some but not in others. 



No observable effect was produced upon the absorption spectra of 

 canary glass, a blue-green variety marked 4102 16 or upon " Didymium" 

 glass by a pressure of 1400 atmospheres. In the last mentioned, 

 the didymium bands were similar to those observed in concentrated 

 solutions of the salts of this substance and such solutions did not 

 show the marked change with pressure which was observed in the 

 more dilute solutions. 



Orange glass, G 34, was observed, at a pressure of 3700 atmospheres, 

 to become more yellowish in color due to a shifting of the transmission 

 band which is in the red end of the spectrum, toward the shorter wave- 

 lengths. Orange glass, G 36, has a similar color change, becoming 

 more yellowish under 1300 atmospheres pressure The single trans- 

 mission band is cut off more on the red side than on the violet side 

 by an increase of pressure. A red glass, G 20, has a much stronger 

 transmission band at high pressures than at low and the band is 

 extended toward the shorter wave-lengths. That the intensity of the 

 transmitted light is increased at high pressure can easily be observed 

 both visually and photographically. 



A comparison of the changes in absorption due to pressure with 

 those due to variation in temperature shows that an increase of pres- 



l 6 The numbers given are those sent by Dr. Gage with the different specimens 

 of glass. 



