Pietenpol—Visible Spectrum of Wisconsin Lake Waters 565 
the selective absorption of water. They studied the relation 
between molecular absorption and the absorption by ions which 
are formed when these molecules dissociate. A concentrated 
solution of a salt contains very many molecules in comparison 
to the number of ions. Dissociation takes place with dilution 
and the number of ions increases at the expense of the number 
of molecules present. Taking comparison absorption spectra 
of solutions of different concentration but with the same num¬ 
ber of parts of solute present it was found that ions have some¬ 
what different absorbing powers from molecules. The more 
concentrated the solution the greater is the absorption and the 
broader and less intense are the absorption bands, which are 
also displaced towards the shorter wave-lengths. An interest¬ 
ing result of this work is the effect of the dissolved substance 
on the spectra of water. Certain aqueous solutions of hy¬ 
drated salts are more transparent than pure water, the com¬ 
bined water having less power to absorb light than free or un¬ 
combined water. An aqueous solution of a non-hydrated salt, 
provided the salt itself does not absorb light, has nearly the 
same absorption as does pure water having the same depth 
as the water in the solution. For a strongly hydrated salt 
the absorption is in general much less than the 'corresponding 
layer of water.^ 
Selective absorption is without question an electronic 
resonance phenomenon. Electrons having definite periods of 
vibration give the media the power to absorb radiations, the 
frequencies of which agree with those of the electrons. The 
period depends upon the chemical constitution or the arrange¬ 
ment of the atoms in the medium. Atoms existing as ele¬ 
ments or in certain compounds may show an absorption very 
different than when in complicated organic compounds, as in 
the case of the aniline dyes where the selective absorption is 
very great. The part the electron plays seems important as 
shown in the effect of dilution upon absorption. 
3. Apparatus and Methods 
The method employed was an optical one, the relative inten¬ 
sities of two spectra at specific wave-lengths giving a measure 
of the absorption of the water. The spectrophotometer used 
^ E. J. Schaeffer, M. G. Paulus; H. C. Jones. 
Phys. Zeitsch., Vol. 15, pp. 447-453, 1914. 
