62 ABSORPTION SPECTRA OF A NUMBER OF SALTS 



Remembering that the solutions with which we were then working, i. e., 

 solutions of salts of neodymium and praseodymium, were strongly hydrated, 

 it was thought that in view of the fact that at least a part, and in the con- 

 centrated solutions a considerable part of the water present was there as 

 water of hydration, it would be advisable to study the effect of colorless 

 hydrated salts upon the absorption of water. 



This chapter of our work has been sufficiently discussed elsewhere in this 

 monograph, and will be taken up here only to state that these experiments 

 showed clearly that there were many variables to be considered. We have, 

 first, the effect of the solvent on the absorption of the solute; and, secondly, 

 the effect of the solute upon the absorption of the solvent. In addition to 

 these, there was, of course, the absorption of the solvent and the solute inde- 

 pendently. Such being the case, we would not be obtaining comparable 

 results for various dilutions of any solutions in terms of Beer's law, even if 

 we did compare each dilution with an equivalent amount of water. It is 

 clear that by so doing we would not be getting comparable ratios, since the 

 solvent and the solute were mutually affecting each other's absorption ; and 

 this effect would not be the same for the different dilutions of the same salt. 



MODE OF PROCEDURE. 



It is, however, possible to get the exact transmission of a given depth of 

 solution by a method of differentiation. If we placed in cell A 11 mm. of a 

 solution and in cell B 1 mm. of the same solution, the ratio representing the 

 respective deflections of the instrument, when these cells are alternately 

 placed in the path of the beam of light, should give the absorption or trans- 

 mission of (11 1) or 10 mm. of the solution. 



Since, if we let A be the percentage absorption of a unit's depth of layer of 

 the solution, and 7 the initial intensity of the light impinging upon the sur- 

 face, we get 



AIq amount of light absorbed by first unit layer of the solution. 

 Then, 



I I A = io(l A) = light incident upon surface of second unit layer. 

 Denoting this by 7i, we get 



7 1 = 7o-/oA=7 (l-A)or^ = l-^ 



Considering again the third unit layer, we get, by similar reasoning, 



IiIiA = amount of light incident upon its surface. 



Denoting this by 7 2 , we get 



h=I 1 -I l A=I 1 (l-A) 



but7 1 = 7 (l-/l); therefore, 7 2 = 7 (1-A) 2 ; hence 7 = I (1-A) n . We can 

 then, by this process, obtain transmissions for given depths of solution 

 and for varying concentrations. This was the method adopted throughout 

 this chapter of the work. 



