DISCUSSION OF EVIDENCE. 185 



WORK OF JONES AND GUY ON THE ABSORPTION SPECTRA OF SOLUTIONS. 



The work on the absorption spectra of solutions had, at the time 

 that Guy began his investigation, been extended to between 6,000 and 

 7,000 solutions. In all of this work the grating spectroscope had been 

 used, and the results recorded on a photographic plate. The photo- 

 graphic method recorded the positions of the various absorption lines 

 and bands, but gave only a qualitative, or at best a roughly quantita- 

 tive indication of the relative intensities of the various lines and bands. 

 The photographic method is, generally speaking, a qualitative method. 



If we are ever to discover relations of fundamental significance 

 between the power of dissolved substances to absorb light and the 

 nature of solution, we must have some quantitative method of study- 

 ing the intensities of the absorption lines and bands and of the various 

 parts of the same bands. With this idea in mind a very sensitive radio- 

 micrometer was built and used to measure the intensity of absorption. 



Before taking up this problem, Jones and Guy investigated two others 

 by the photographic method. They studied the effect of temperature 

 on the absorption spectra of aqueous solutions up to 200. This 

 required a specially designed apparatus which would not be attacked 

 by the superheated water-vapor. It was found that while some of the 

 bands of aqueous solutions are practically unaffected by rise in tem- 

 perature, many of them widen as the temperature is raised. The 

 widening of the absorption bands is usually not symmetrical, but is 

 generally towards the red. The red edge widens out, becoming more 

 hazy and diffuse, while the violet edge remains pretty sharp. The 

 effect of rise in temperature on the absorption spectra of aqueous solu- 

 tions is, then, often analogous to the effect produced by increasing the 

 concentration of the solution. This is especially the case with solutions 

 of praseodymium nitrate. The effect of dilution on absorption spectra 

 was studied pretty thoroughly by Jones and Guy. It was well known 

 that both molecules and ions can absorb light, and the question was, 

 do they have the same or different absorption? Jones and Anderson 1 

 had shown that if they absorb differently, the difference is slight. To 

 detect any such differences wide ranges in dilution must be employed. 



Cells were devised for holding the solutions, which were 0.5 cm., 

 50 cm., and 250 cm. in length. The concentrations were varied in 

 the same proportions as the lengths of the cells. If we call the con- 

 centration used in the shortest cell unity, 100 times as dilute a solution 

 was used in the cell which was 50 cm. long, and this was diluted 5 times 

 for the longest cell. It was found that many of the absorption bands 

 of neodymium chloride and bromide widen as the concentration of the 

 solution is increased. Some of the bands of neodymium sulphate and 

 acetate show similar changes with increase in the concentration of the 

 solution. The most marked changes, however, are produced with the 

 bands of neodymium nitrate. Many of them show very pronounced 



Carnegie Inst. Wash. Pub. No. 110. 



