66 ABSORPTION SPECTRA OF SOLUTIONS. 



of aluminium chloride in the solutions used in making B were, taken in 

 the same order, 2.58, 2.25, 1.86, 1.50, 1.14, 0.78, 0.42, and 0.0. 



The common depth of absorbing layer was 1.5 cm., and the times of 

 exposure to the light of the Nernst lamp and spark were, respectively, 

 1J and 3 minutes; the slit had the usual width of 0.01 cm. 



The effect of adding these dehydrating agents is to widen all the absorp- 

 tion bands, the widening seeming to increase a little more rapidly the 

 more concentrated the solution of the dehydrating agent. This gives the 

 edges of the bands a slightly rounded appearance. 



It is also noticed that the effect of the aluminium salt is a little greater 

 than that of the calcium salt, although the concentrations were so chosen 

 as to make the number of chlorine atoms added as nearly the same as 

 possible. The widening can not be due to a driving back of the dissocia- 

 tion of the chromium salt, since we have just seen that the absorption of 

 the chromium chloride does not in any way vary with its dissociation. The 

 most probable explanation here as elsewhere is that some simple hydrates 

 are formed, which normally do not exist except in very concentrated solu- 

 tions or at high temperatures, and that the absorbing power of a chromium 

 atom thus hydrated is greater than when the hydrate is more complex. 



The band in the red at ^6690 shows faintly on the negatives for both these 

 spectrograms, although it can not be seen in the reproduction. It does not 

 seem to be affected to any appreciable extent by addition of the foreign salts. 



CHROMIUM NITRATE IN WATER BEER'S LAW. (See Plate 58 A.) 



The concentrations of the solutions, beginning with the one whose 

 spectrum is adjacent to the numbered scale, were 0.754, 0.564, 0.377, 0.251, 

 0.174, 0.126, and 0.094; the corresponding depths of absorbing layer were 

 3, 4, 6, 9, 13, 18, and 24 mm. The exposures to the light of the Nernst 

 lamp and spark lasted, respectively, 1J and 3 minutes, the slit having the 

 usual width of 0.01 cm. 



The solutions of the nitrate are similar in color to those of the chloride 

 already described, excepting that the latter is relatively more transparent 

 in the green and less so in the red. The result is that in layers of any 

 depth the nitrate solutions are more apt to show red, especially in gas- 

 light. Dilute solutions or very thin layers of concentrated solutions are 

 greenish in color. 



The spectrogram shows the same absorption bands as we have already 

 found and discussed for the chloride. Owing to the fact that the concen- 

 tration of the solutions of the nitrate was somewhat greater, the bands 

 are wider and their edges are much sharper. 



In the ultra-violet the transmission is sharply limited by the N0 3 band, 

 and hence we find absorption complete from ^ 3270 to the end of the 

 spectrum. 



In the most concentrated solution, the violet band begins at A 3710 

 and ends at A 4450, these figures being the limits of (photographic) trans- 

 mission. Owing to the slight shading the absorption extends somewhat 

 farther to both sides. In the most dilute solutions the limits are X 3710 

 and A 4420, showing a slight narrowing of the band from the red side. 



