32 THE ABSORPTION SPECTRA OF SOLUTIONS. 



much success. According to the theory of aggregates, however, it would not 

 be expected that any very marked effect would result, unless the two salts 

 formed parts of the same aggregate; the term aggregate being used in the 

 broader sense here to include also molecules of the solvent. Accordingly, the 

 list of colored chemical compounds was searched for salts that contained 

 colored anions and colored cations. An effort was also made to obtain solvents 

 and salts that had bands in the same part of the spectrum. So far no good 

 examples were found where the same spectral aggregates contained different 

 absorbing centers that possess bands in the same region of the spectrum. It 

 seems quite possible, however, that aggregates of this kind among organic 

 compounds could be found, or that in the infra-red region many examples of 

 this kind will be found. The region of the infra-red is especially inviting for 

 work of this kind, inasmuch as nearly all groups possess bands, and the 

 absorbers seem to be of molecular dimensions; whereas in the visible portion 

 of the spectrum it seems probable that, in most cases at least, the constitution 

 of the spectral aggregate only influences the period of vibrators that seem to 

 be of the nature of electrons. 



It has been shown that the presence of calcium or aluminium chloride has 

 a considerable influence upon the wave-lengths of the uranyl chloride bands. 

 This has been explained 1 as being due to the presence of chlorine, rather than 

 to the direct presence of the calcium or aluminium atoms. It is possible to 

 obtain aqueous solutions of calcium ferricyanide or of aluminium and calcium 

 chromate. If the change of wave-lengths of the uranyl bands was not due to 

 calcium and aluminium, then the absorption of the potassium, aluminium, and 

 calcium ferricyanides and chromates should be the same. These salts were 

 studied to test whether or not this was the case. 



The dissociation 2 of the ferricyanides, ferrocyanides, and chromates is 

 an interesting one, and the study of the absorption spectra of solutions of 

 these salts, especially in the infra-red, will probably throw much light upon 

 this subject, but in this work time did not permit us to take up this problem. 



Calcium Ferrocyaxide and Calcium Ferricyanide. 



The absorption spectrum of calcium ferricyanide is given in plate 2, A, 

 and of calcium ferrocyanide in plate 2, B. The concentrations are given in 

 the chapter on the description of plates. Starting with strip 1 of A, the edge 

 of the absorption band is at about X 4700; the concentration (c) being 0.031 

 normal and the depth of cell (d) 24 mm. (hereafter the product cd will be 

 given without definition, c being expressed in terms of normal and d in milli- 

 meters), the value of cd being 0.74. In the case of potassium ferricyanide 3 

 for a value of cd of 0.69 the edge of the absorption band comes at about 

 X4690, considering the limit of absorption as at X4710 and the distance be- 

 tween this limit and the place where the absorption is 50 per cent as being 20 

 Angstrom units. It is thus seen that the absorption of calcium ferricyanide 

 is approximately the same as that of potassium ferricyanide. The conclusion 

 follows that calcium shows no bathochromous effect in this instance. 



1 I'hys. Zeit., II, 668 (1910). 



- Publication 130, Carnegie Institution of Washington, 28 ami 29. 



:i Ibid., 29. 



