POTASSIUM SALTS. 29 



spectra of either FeC 6 N 6 , FeC c N 6 , FeC 4 N 4 , or FeC 3 N 3 , then we could tell 

 whether the salts dissociated according to the first or the second theory, or 

 according to some other method. But as we have thus far investigated 

 only the absorption spectra of potassium ferricyanide and potassium ferro- 

 cyanide, no conclusion as to the manner of dissociation could be drawn. 



It would be interesting in this connection to study potassium ferro- 

 cyanide carbonyl, K 3 FeC 5 N 5 CO:3H 2 0, ferrocyanhydric acid, H 4 FeC 6 N, 

 and ferricyanhydric acid, H 3 FeC 3 N 3 . Both ferrocyanhydric acid and 

 ferricyanhydric acid are soluble in water and alcohol. 



The absorption spectra of a 3-mm. depth of cell of potassium ferri- 

 cyanide in water were photographed between concentrations 1 normal and 

 0.00024 normal, and are given in Plates 10 and 11. The conditions of 

 exposure in these two plates were the same, 80 seconds to the Nernst glower 

 for the visible portion of the spectrum and 2 minutes to the spark for the 

 ultra-violet region. The current through the Nernst glower was kept 

 constant at 0.8 ampere. The slit-width was 0.08 mm. 



Potassium ferricyanide has a very simple absorption spectrum. For the 

 concentrated solutions all the short wave-lengths beyond the green are 

 absorbed. As the concentration is decreased the limit of absorption grad- 

 ually recedes towards the ultra-violet. The region between complete trans- 

 mission and complete absorption is quite narrow for the more concentrated 

 solutions, and for this reason solutions of potassium ferricyanide could be 

 used as light-screens. The concentrations starting at the top of A, Plate 10 

 are 1, 0.75, 0.50, 0.333, 0.231, 0.166, and 0.125 normal, the corresponding 

 limits of absorption being XX 4800, 4780, 4765, 4730, 4710, 4680, and 4650. 

 The distance between complete absorption and complete transmission as 

 measured on the spectrum photograph was not greater than 40 Angstrom 

 units. 



The concentrations starting at the top of B, Plate 10 are 0.125, 

 0.0937, 0.0625, 0.0417, 0.029, 0.0208, and 0.0156 normal. The correspond- 

 ing limits of absorption are XX 4650, 4630, 4620, 4600, 4560, 4520, and 4500. 

 As the concentration becomes small the limit of absorption becomes more 

 and more diffuse. For the 0.0156 normal concentration the distance between 

 complete transmission and complete absorption was almost 100 Angstrom 

 units. For about this concentration a faint transmission band begins to 

 appear in the ultra-violet. 



The concentrations starting at the top of A, Plate 11 are 0.0156, 

 0.0117, 0.0078, 0.0052, 0.0036, 0.0026, and 0.00195 normal. In this spec- 

 trogram two bands of absorption appear, a blue-violet band and an ultra- 

 violet band. For the 0.0156 normal solution the blue-violet absorption 

 band is bounded by X 450G and X 3570, for 0.0117 normal by XX 4450 and 

 3580, for 0.0078 normal XX 4400 and 4000. The middle of the band at its 

 origin is about X 4200. The long wave-length edge of the ultra-violet band 

 has the following positions, for normal 0.0156, X 3550; 0.0117, X 3500; 

 0.0078, X 3300; 0.0052, X 3250; and 0.0036, A 3150. For concentrations 

 less than this there is more or less general transmission throughout the 

 ultra-violet region. 



