SALTS OF COBALT. 21 



percentage relation of water to alcohol, but of water to amount of salt in 

 a given volume of the solution. This was found to be so for solutions of 

 neodymium salts in mixed solvents, which will be discussed in the chap- 

 ter dealing with the rare earths. In the event of this rule also applying 

 to cobalt salts, we may say that since 5 per cent of water produces a fair 

 development of the bands in a solution of concentration 0.088, it would 

 require only about 1 per cent of water to do so for the most dilute solu- 

 tion used in making the negative for Plate 5, where the concentration 

 was 0.02. The bands then were much less clearly developed, which in the 

 event of their being caused by water would indicate the presence of 

 water to an amount of say 0.5 per cent, which is well within the range of 

 probability. 



It is not at all improbable that the absorption bands in the red region 

 of the spectrum, shown by solutions of cobalt salts dissolved in ethyl alcohol, 

 could be used as a delicate test for the presence of water in the solvent. 



COBALT CHLORIDE IN ACETONE WITH WATER. (See Plate 9.) 



The concentration of the cobalt salt throughout was 0.0108. The 

 successive percentages of water were, beginning with the solution corre- 

 sponding to the strip nearest the numbered scale, 0, 2, 4, 6, 8, 10, 12, 14, 

 16, 18, 20, 22, 24, 26, 28, and 30. 



The solutions containing from to 10 per cent of water were deep-blue 

 to light-blue, while those containing from 16 to 30 per cent of water showed 

 very little color except a faint suggestion of pink. The duration of the 

 exposure to the Nernst lamp lasted 1 minute, that to the spark 3 minutes, 

 with a slit width of 0.01 cm. The cell was adjusted to a depth of 3.0 cm. 



The intense absorption in the ultra-violet is due to the acetone, and 

 hence no transmission is noticed beyond A 3300. The solutions contain- 

 ing the least amount of water show a region of absorption near X 3600, 

 which may be the same band that has already been described in discussing 

 solutions in the alcohols. The solutions containing from to 14 per cent 

 of water show absorption in the orange and red, which decreases rapidly 

 with increase of the amount of water. 



The solution in pure acetone absorbs everything from X 5400 to A 7300. 

 Both edges of the band approach each other rapidly until the solution 

 containing 10 per cent of water is reached, when it breaks up into a number 

 of narrower bands, their centers being at X 5910, A 6060, A 6240, and A 6700. 

 The band at A 6400 ; in the ethyl alcohol solution on addition of water, does 

 not appear distinctly enough to be seen with certainty. In general, the 

 system of bands is the same as that which is seen in ethyl alcohol, and 

 is most likely due to the presence of water; since it will be recalled, from 

 the description of Plate 6, that the solution in pure acetone shows no 

 system of bands at all comparable with those here described. The pro- 

 portion of water to salt in the solution, in order to show the bands, is 

 very much larger for solutions in acetone than for solutions in ethyl alco- 

 hol, the values being about as 15 to 1. 



The change in the spectrum produced by adding more water than 14 

 per cent is too slight to be noticed. 



