206 HYDRATES IN AQUEOUS SOLUTION. 



blue. The photographic strips succeed one another from the side of the 

 spectrogram adjacent to the numbered scale, to the edge contiguous to the 

 spark spectrum, in the order of increasing concentration of the dehydrating 

 agent. The depth of the cell was 1.41 cm. The spectrogram shows quali- 

 tatively the same regions of absorption as have been noted for all of the 

 preceding series or sets of solutions containing cobalt chloride. 



The negative recorded the line at 2265.1 as the most refrangible radiation 

 transmitted by the solution which contained none of the aluminium chloride. 

 The most concentrated solution transmitted a trace of the line of wave- 

 length 2748.7, but the continuous background was completely absorbed 

 beyond 0.288. The absorption band in the green was at about 0.518/. The 

 greatest wave-length for light transmitted at the blue side of this band, 

 for the most concentrated solution, was given by the negative as 0.4S4/1. 

 The seventh and eighth photographic strips, especially, show general absorp- 

 tion in the orange. 



A few attempts were made to find out whether any simple relation might 

 exist between two solutions having apparently the same color, the one con- 

 taining calcium chloride and the other aluminium chloride. Except for the 

 extremely concentrated solution, it was found that as a first approximation 

 two solutions of the kind just mentioned were isochromatic when they con- 

 tained equal numbers of chlorine atoms. The number before each comma 

 in the following sequence denotes the concentration of the solution contain- 

 ing the chlorides of cobalt and calcium, and the number immediately follow- 

 ing the comma in question signifies the concentration of the corresponding 

 solution containing the same amount of cobalt chloride, and such an amount 

 of aluminium chloride as to possess the same number of chlorine atoms as 

 the solution whose concentration precedes the comma. The pairs are: 1.676, 

 1.118; 2.091, 1.394; 2.515, 1.676; 2.671, 1.781; 2.830, 1.887; 3.143, 2.100; 

 and 3.555, 2.370. As far as the eye could tell the members of each pair of 

 solutions had the same color when they were viewed in their bottles. All 

 of the calcium chloride solutions and all of the aluminium chloride solutions, 

 except the most concentrated one, have their absorption spectra reproduced 

 by plates 9 (a) and 11 (6), respectively. To make the comparison easier and 

 more direct, the members of the first, third, sixth, and last pairs of solutions 

 were photographed side by side and in quick succession. The resulting 

 spectrogram is shown on plate 12. Counting from the side of the plate 

 nearest to the numbered scale, the first, third, fifth, and seventh photographic 

 strips correspond to the solutions which contain calcium chloride, and the 

 remaining strips pertain to the solutions which contain aluminium chloride. 



The depth of the cell was 1 cm. In the ultra-violet the solutions which 

 contained the aluminium salt showed much stronger absorption than those 

 which were made with the calcium salt. On the other hand, the band in the 



