32 J. G. MACGEEGOE ON THE 



provided the ooncentratious of the solutions are inversely as the thicknesses of the strata ; 

 for according to this hiw absorption does not vary with concentration alone. But the 

 variation with temperature is small. And Beer's law, though confirmed as practically 

 accurate by the experimental investigations of many eminent observers, may not be rigor- 

 ously accurate. The above conclusions may therefore be borne out by the small varia- 

 tions of absorption with concentration, which, though not of sutficient magnitude to have 

 been hitherto of importance in the applications which have been made of Beer's law, as, 

 e. g., to quantitative spectrum analysis, may nevertheless be real. 



The experience of those who have worked in the department of quantitative spectrum 

 analysis would seem to indicate that its methods are becoming so accurate that the merely 

 approximate character of laws hitherto assumed to be exactly applicable must be taken 

 into account. Thus Gr. and H. Krliss ' point out that the " absorption ratio " of a solution 

 for light of a given wave length must no longer be assumed to be independent of the 

 concentration. The absorption ratio is the quotient of the extinction-coefficient of a solu- 

 tion by its concentration. The extinction-coefficient is the reciprocal of the thickness 

 which the absorbing solution must have in order that the transmitted light may have its 

 intensity diminished to one-tenth of that of the incident light. It is calculated from the 

 observed absorption of any convenient thickness of the solution by the application of 

 Lambert's law of the relation of the absorption to the thickness of transparent bodies. 

 It follows from the definition of the extinction-coefiicient that, if both Lambert's and Beer's 

 laws hold, the absorption ratio must be independent of the cogcentration. That it is 

 not, shows that one or other or both, must be merely approximately true. It would be 

 difficult to say which law is best supported by experimental evidence. But as the former 

 assumes only that a given stratum of a given solution absorbs the same fraction of the 

 lio-ht of given wave length which passes through it, whatever be the intensity of the in- 

 cident light, and as the latter involves the assumption that the absorption of light by a 

 solution is not affected by the action between the salt molecules and those of the solvent, 

 it would seem to be probably the approximate character of the latter to which the vari- 

 ability of the absorption ratio with concentration is due. 



So far as I am aware no systematic observations have been made to determine the re- 

 lation between the effects of elevation of temperature and increase of concentration on 

 the absorption of light. I have myself been able to make so far only rough qualitative 

 observations, which, though they are in accordance with the conclusions reached above, 

 are not worth publishing. A few isolated observations of a quantitative kind, however, 

 have been made, which, though made for other purposes and by différent experimenters, 

 may, when combined, serve to test the above conclusions in a more or less rigorous 



manner. 



Russell - has examined the absorption spectra of aqueous solutions of cobalt chloride 

 and cobalt bromide at different concentrations and temperatures. He found that if, to a 

 solution of the chloride (CoCL), formed by adding 4-18 grm. of the salt to 10 ccm. of 

 water at 0^ C, water be added in various quantities up to 16 ccm., the dilute solutions 

 thus formed may be made to give the same absorption spectrum as the original solution 



G. and H. Kriiss: ' Kolorimetrie,' Hamburg (1891), p. 146. 

 ' ' Chem. News,' vol. li (1885), p. 259 ; ' Beiblatter^Wied. Ann.,' B.l. x (1886), p. 570. 



