34 



J. G. MACGREGOE ON THE 



I liave made the calculations necessary for this purpose ; and in the tables given below the 

 intensities of transmitted light are for such thicknesses of the soIi;tions compared that 

 the product of thickness into concentration is constant. These tables, therefore, show the 

 variation of absorption with concentration. The intensities of transmitted light are in all 

 cases expressed as fractions of those of the incident light. The regions of the spectrum exa- 

 mined are indicated by the terminal and mean wave lengths in million ths of a millimetre. 



It should be noted that the calculations by which the intensities of light for the more 

 dilute solutions of the following tables have been determined, necessarily increase the 

 errors of the original observations in the ratio of the concentrations of the solutions com- 

 pared. The intensities given for dilute solutions are thus in all cases less accurate than 

 those for strong solutions. No small effects can therefore be regarded as established by 

 the relative values of the intensities of the following tables. But such small effects may 

 be indicated in this way. And any nniformity in the small effects indicated may serve 

 to establish a, prima facie case, and to show at any rate that a direct experimental study of 

 the subject is desirable. 



The first table shows the absorption at both ends of the absorption baud extending 

 from about waA'e length 590 to wave length 460 in solutions containing O'OOl and 000025 

 grm. of the salt per cu. cm. : — 



If these results be treated graphically, curves being drawn for both solutions with the 

 mean wave lengths of the regions of the spectrum examined, as abscissœ, and the inten- 

 sities of light as ordinates, it will be observed (1) that at the red- ward end of the band 

 the curves for both solutions seem to cut the axis of intensities at the same point ; (2) that 

 at the red-ward end the curve for the weaker solution is steeper than the curve for the 

 stronger solution, and (3) that at the violet-ward end of the band the one point which the 

 experiments determine for the strong solution lies almost exactly on the curve for the 

 weak solution. In other words the violet-ward end of the band seems to be unchanged 

 by the increase of concentration under consideration ; at the red-ward end the position of 

 the boundary of complete absorption seems also to be unchanged, but the portion of the 

 band in which the absorption is incomplete, which we may call for shortness its pennm- 

 bra, seems to have extended somewhat towards the red. 



