34 ABSORPTION SPECTRA OF SOLUTIONS. 



A comparison of the first strip of B with the first strip of Plate 20 A 

 shows that the width of the green band in the two is very nearly the same. 

 The depth of cell used was 3 mm. in each case, but the concentration of 

 the nitrate was 2.05 while that of the sulphocyanate was only 0.51, indi- 

 cating an absorbing power for the sulphocyanate nearly 4 times as great 

 as for the nitrate. 



COBALT ACETATE IN WATER BEER'S LAW. (See Plate 24.) 



The concentrations of the solutions used in making the negative for 

 A, beginning with the one whose spectrum is nearest the numbered scale, 

 were 0.86, 0.68, 0.54, 0.43, 0.34, 0.27, and 0.22; the corresponding depths 

 of absorbing layer were 3, 4, 6, 9, 13, 18, and 24 mm. For B the concen- 

 trations were 0.34, 0.25, 0.21, 0.17, 0.14, 0.11, and 0.09; the depths of 

 cell were the same as in A. The color of the solutions changed from pur- 

 plish to the usual pink with increasing dilution. The exposures to the 

 Nernst lamp and spark were 1 and 2 minutes respectively, the slit having 

 a width of 0.01 cm. as usual. 



The absorption in the ultra-violet is slight, and decreases with decrease 

 in concentration. The limits of transmission for the most concentrated 

 and most dilute solutions of set A are X 2620 and X 2470, and for set B 

 X 2470 and X 2390. Some irregularities in the intensities of the various 

 spark spectra are noticed, indicating considerable fluctuations in the 

 intensity of the spark during the different exposures. In spite of this 

 there can be no doubt that the spectrogram shows an increase of transpar- 

 ency with decreasing concentration. 



The green band narrows with dilution, but more and more slowly as 

 the concentration is diminished. In the last three or four solutions of 

 set B it remains practically constant. The limits of transmission for the 

 most concentrated and the most dilute solutions of set A are X 4500 and 

 X 5600, and X 4600 and X 5520, respectively, placing the center of the band 

 at about X 5050 throughout. In set B the center is near ^5175. A part 

 of this change in position may, however, be accounted for by the lack 

 of uniformity in the sensibility curve for the Seed film. From the red 

 edge of the green band the solutions transmit freely light of all wave- 

 lengths as far as beyond X 7400. 



Strip 1 of A may be compared with the first strip of Plate 11 A, and 

 it will be found that the width of the green band in the two is very nearly 

 the same. The depth of the absorbing layer in each case was 6 mm., but 

 the concentration of the bromide solution was 1.51, while that of the 

 acetate was 0.86; thus showing that the acetate solution absorbs green 

 light much more strongly than a solution of the bromide having the same 

 concentration. A comparison of the seventh strip of A with the seventh 

 strip of Plate 23 B shows that the green bands in the two agree almost 

 exactly in width and position. The depth of the cell in each case was 

 24 mm., while the concentrations of the sulphocyanate and acetate were, 

 respectively, 0.10 and 0.22. This indicates that the sulphocyanate solu- 

 tion has about double the absorbing power of the acetate solution of equal 

 concentration, for green light. 



