426 Prof. Baly and Mr. Tryhorn on Light 



According to the well-known law the absorptive power is 

 expressed by the relation 



I ~ 6 ' 



where I and I are the intensities of the incident and emer- 

 gent light respectively, d the thickness of the absorbing- 

 layer, c the concentration, and k the absorption constant.. 

 In actual experiment the absorption is best expressed in 

 terms of that exerted by 1 cm. of a molar solution (one 

 molecular weight dissolved in one litre of solution). The 

 value of k, which according to Beer's law should be constant,, 

 is readily enough found from 



logl 9 / 



Xdc = k. 



In practice, however, it is rarely found that Beer's law holds 

 good, but up to the present no expression has been found 

 which connects the value of k with the concentration. Some 

 previous observations of the absorptive power of aqueous 

 solutions of pyridine * have shown qualitatively that it 

 increases very materially as the concentration decreases.. 

 An interesting fact in connexion with pyridine is that its 

 absorptive power is much greater in acid solution than in 

 neutral solvents t. This at once affords a considerable range 

 of solvents, and we have determined quantitatively the value 

 of k for pyridine solutions in the following solvents — water, 

 alcohol, N/5000 aqueous hydrochloric acid, N/1 aqueous 

 hydrochloric acid, and concentrated sulphuric acid. In the 

 case of the neutral solvents, considerable care was necessary 

 to guard against the absorption of atmospheric carbon 

 dioxide which at once gave too high values for k. In the 

 case of each solvent, the absorptive power of pyridine in- 

 creases rapidly with the dilution until a constant value is 

 reached, and the values observed are given in Table IV. 



In previous papers J the view has been put forward that 

 the molecules of every substance are surrounded by a more 

 or less condensed force field of electromagnetic type. These 

 fields are opened or unlocked by the influence of a solvent, 

 such opening up taking place in definite stages. Each of 



* Baly and JRice, Trans. Chem. Soc. ciii. p. 91 (1913). 



t Hartley, ibid, xlvii. p. 685 (1885), and Baker and Baly, ibid, xcu 

 p. 1122 (1907). 



X Balv and Krulla, Trans. Chem. Soc. ci. p. 1469 (1912) ; Baly and. 

 JRice, ibid, ci. p. 1475 (1912), ciii. pp. 91 & 2085 (1913) ; and Baly, Phil. 

 Mag. xxvii. p. 632 (1914), xxix. p. 223 (1915). 



