198 JONES-STRONG— THE ABSORPTION SPECTRA [April 24. 



II. Absorption Spectra of Potassium Salts in Aqueous 



Solutions. 



Most potassium salts in solution are colorless, and for this reason 

 it is considered that the potassium atoms do not themselves absorb 

 any light in the visible portion of the spectrum. Several colored 

 potassium salts are known and the color of these is due in some 

 way to the other atoms in the salt molecules. In the present work 

 the absorption spectra of potassium ferricyanide, potassium ferro- 

 cyanide, potassium chromate and potassium dichromate have been 

 studied. 



Using a 3 mm. length of solution of potassium ferricyanide in 

 water we find that for a normal concentration there is complete 

 absorption of all the shorter wave-lengths of light beyond A. 4800. 

 As the concentration is decreased the edge of transmission moves 

 continually towards the violet. It should be noticed that the 

 region between complete absorption and complete transmission for 

 the more concentrated solutions is quite narrow, being less than 40 

 Angstrom units; thus making solutions of this salt quite good 

 screens for absorbing light. Continually decreasing the concentra- 

 tion we reach a 0.0156 normal solution, when a transmission band 

 begins to appear. For a certain range of concentration there ap- 

 pears an absorption band in the region A 4200. Further decrease in 

 concentration results in increasing transmission throughout the 

 violet and ultra-violet. For dilutions greater than 0.00195 normal 

 there is almost complete transmission throughout the ultra-violet. 

 Very faint bands appear in the regions AA2500 to 2600, AA2950 to 

 3050 and AA3200 and 3250. 



Several spectrograms were made, keeping the product of con- 

 centration and depth of solution layer constant. In this case the 

 spectrograms will be identical if Beer's law holds. Beer's law was 

 found to hold according to this method of testing within the ranges 

 of concentration over which the spectrum was mapped. 



The absorption of aqueous solutions of potassium ferrocyanide 

 was investigated in the same way. A half-normal solution 3 mm. 

 deep shows that all light of shorter wave-length than A 3950 is 

 absorbed. Keeping the depth of layer the same, it is found that 



