30 A STUDY OF THE ABSORPTION SPECTRA. 



The concentrations for B, Plate 11, starting with the strip nearest the 

 top, were 0.00195, 0.00146, 0.00098, 0.00065, 0.00045, 0.00037, and 0.000244 

 normal. In this spectrogram there is somewhat general transmission 

 throughout the whole ultra-violet region. In the ultra-violet there appear 

 three weak bands that do not seem to be due to any feeble intensity of the 

 spark-spectra in these regions. The positions of these bands are X 2500 

 to 2600, X 2950 to 3050, and X 3200 to 3250. 



Spectrograms were made to test Beer's law between concentrations 

 1 and 0.000244 normal, four spectrograms being made in all. In each one 

 of these spectrograms the amount of absorbing salt in the solution through 

 which the light passed, times the length of the solution was constant. 

 Under these conditions if Beer's law holds the absorption spectra in each 

 strip will be the same. 



Plate 12, A represents a spectrogram showing that Beer's law holds 

 for a potassium ferricyanide solution between concentrations 1 and 

 0.125 normal. Starting with the strip at the top the concentrations were 

 1, 0.75, 0.50, 0.333, 0.231, 0.166, and 0.125 normal. The depths of cell 

 were 3, 4, 6, 9, 13, 18, and 24 mm., respectively. The exposure to the 

 Nernst glower was made for 80 seconds, slit-width being 0.08 mm. and the 

 current 0.8 ampere. The limit of absorption for each strip is X 4800. B, 

 Plate 12, was taken under the same conditions as A t each strip, however, 

 being exposed to the spark for 3 minutes. The concentrations for B, start- 

 ing with the upper strip, were 0.125, 0.094, 0.0625, 0.0417, 0.029, 0.0208, 

 and 0.0156 normal, the depths of cell being 3, 4, 6, 9, 13, 18, and 24 mm. 

 In this case also Beer's law held to within the limits of error of this method. 



Spectrograms not published were made in a similar way for concentra- 

 tions 0.0156, 0.0117, 0.0078, 0.0052, 0.0036, 0.0026, and 0.00195 normal; 

 also for concentrations 0.00195,0.00146, 0.00098, 0.00065, 0.00045, 0.00037, 

 and 0.000244 normal, the depths of cell being 3, 4, 6, 9, 13, 18, and 24 mm., 

 respectively. In all these cases Beer's law holds. 



A question that is sometimes asked is, What is the sensitiveness of this 

 method to small changes in concentration ? This depends of course on the 

 absolute concentration itself. For concentrated solutions of potassium 

 ferricyanide it is possible to detect a change in the edge of the absorption 

 to within 10 Angstrom units. For concentrations of 0.125 normal, then, it 

 would be possible to detect changes of concentration of less than normal 

 0.02. It will be noticed that the relative sensibility to change in concentra- 

 tion is not very great, but for dilute solutions the absolute sensibility is 

 much greater. One reason for this is that errors in the depth of cell for a 

 considerable cell-depth are small as compared with the depth of cell itself. 



