18 ABSORPTION SPECTRA OF SOLUTIONS. 



MAKING A DILUTION SPECTROGRAM. 



Before entering upon a detailed discussion of the spectrograms, it is 

 wise to state briefly the method used in making any given spectrogram. 

 Throughout all the work done on the effect of high dilution on absorption 

 spectra, under the conditions of Beer's law, only three exposures were made 

 for any given spectrogram, i. e., only three dilutions were compared. The 

 depths of cell in all cases were 0.5 cm., 50 cm., and 250 cm., the dilution being 

 increased 100 times between the first two solutions and 5 times between the 

 last two; or a total dilution of 500 times between the first and last solution. 

 Smaller depths of cell than 5 mm. were not used, on account of the large 

 percentage error in measuring such depths. 



Much difficulty was experienced in getting sufficient light through the 

 longer cells to fill the grating completely; nor was this possible unless the 

 tube containing the solution was constantly moved backwards and forwards 

 so that the image of the source of light was moved along the slit of the camera. 

 By such a procedure the surface of the grating could be illuminated fairly 

 uniformily, and the exposures gave good results on the photographic plate, 

 as is shown by the spectrograms. 



In order to insure complete illumination of the grating a uniform pro- 

 cedure was adopted. The longest cell, containing the most dilute solution, 

 was first placed in position, the light passed through, and the image of the 

 Nernst glower sharply focused on the slit of the camera in such a manner as 

 to throw as much light as possible on the grating. By holding the eye in the 

 position later to be occupied by the photographic plate, we could easily tell 

 when the grating was properly illuminated. 



After everything was properly adjusted the lights were extinguished and 

 the plate inserted in the camera. Great care was taken not to move any 

 parts of the apparatus, the camera was closed, the source of light again 

 turned on, and the exposure made. It is clearly seen that in making any 

 spectrogram, using three cells differing in length so markedly, we virtually 

 had three different sources of light, and, consequently, the length of exposure 

 sufficient to give comparable results on the photographic plate had to be 

 determined by a long series of trials. In the case of the longest cell, expos- 

 ures as long as several minutes were made, while with the shortest cell only 

 a few seconds were necessary to give good clear spectrograms on the photo- 

 graphic plate. 



The remaining procedure was essentially the same as that described by 

 Jones and Anderson 1 and by other workers in this laboratory. 



NEODYMIUM CHLORIDE IN WATER. (See Plate 13.) 

 The concentrations of the solutions used in making the negative for A, 

 beginning with the one whose spectrum is farthest from the spark spectrum, 

 were 2.05, 0.0205, and 0.00401 normal, respectively, the corresponding 

 depths of absorbing layer being 0.5 cm., 50 cm., and 250 cm. 



iCarn. Inst, Wash. Pub. 110. 



