SALTS OF IRON. 61 



the one containing the calcium salt. For the solution containing ferric chlo- 

 ride, at concentration 0.035, the difference is again 50 A.U., while for the 

 one having the concentration 0.007 the difference amounts to 80 units. 



FERRIC CHLORIDE IN METHYL ALCOHOL BEER'S LAW. (See Plates 52 and 54 A.) 



The concentrations of the solutions used in making the negative for 

 A, Plate 52, beginning with the one whose spectrum is adjacent to the 

 numbered scale, were 1.23, 0.923, 0.615, 0.410, 0.284, 0.205, and 0.135; 

 the corresponding depths of absorbing layer were 3, 4, 6, 9, 13, 18, and 

 24 mm. For B, Plate 52, the concentrations were 0.2, 0.15, 0.1, 0.066, 

 0.046, 0.034, and 0.025, and for A, Plate 54, they were 0.034, 0.026, 0.017, 

 0.011, 0.0078, 0.0057, and 0.0042; the depths of cell were the same as in 

 A, Plate 52. 



The most concentrated solutions were deep orange-red, from which on 

 dilution the color changed to a clear yellow. 



The exposure which was made to the Nernst lamp lasted only 1 minute, 

 the slit having the usual width of 0.01 cm. No exposure was made for 

 the red end of the spectrum, as examination by the direct-vision spectro- 

 scope showed no absorption in this region. 



The three spectrograms show that Beer's law holds very accurately 

 over the range of concentrations studied, the edge of the absorption band 

 remaining unchanged in position in any one series. In A, Plate 52, the 

 limit of transmission is at A 5300, in B at A 4950, and in A, Plate 54, it falls 

 at A 4600. 



In A, Plate 46, the limit of transmission was not far from A 4700. The 

 concentrations and depths of cell there were about the same as in A, Plate 

 52, while the solvent there was water and here methyl alcohol. This indi- 

 cates considerably greater absorbing power for the salt when dissolved 

 in methyl alcohol, if, as is usual, the actual shift of the center of the absorp- 

 tion band is not very great. In the present case, since Beer's law holds, 

 we may assume that all the moving parts containing an iron atom are 

 equally active in absorbing light; while in the aqueous solution, since Beer's 

 law does not hold, some of them must either not absorb at all, or else much 

 more feebly than others. 



FERRIC CHLORIDE IN ETHYL ALCOHOL BEER'S LAW. (See Plates 53 and 54 B.) 



The concentrations of the solutions used in making the negative for 

 A, Plate 53, beginning with the one whose spectrum is adjacent to the 

 numbered scale, were 1.23, 0.923, 0.62, 0.41, 0.28, 0.21, and 0.15; the 

 corresponding depths of cell being 3, 4, 6, 9, 13, 18, and 24 mm. For B, 

 Plate 53, the concentrations were 0.20, 0.15, 0.10, 0.066, 0.046, 0.034, and 

 0.025; and for B, Plate 54, 0.034, 0.026, 0.017, 0.011, 0.0078, 0.0057, and 

 0.0042 ; the depths of absorbing layer were the same as in A, Plate 53. 



The color of these solutions was identical with that of the solutions 

 in methyl alcohol. The exposure, which was made to the light of the Nernst 

 lamp, lasted only for 1 minute ; the slit had a width of 0.01 cm. 



It will be seen that here also Beer's law holds fairly well, the deviation 

 from it in the most concentrated series causing the band to narrow by 



