391 
DiscussIoN AND SUMMARY OF RESULTS 
Present-day methods of determining hydrogen ion concentration in 
detailed manner have been developed by biochemists, bacteriologists, and 
- oceanographers; not by physical chemists, as one might expect. Its im- 
portance in the study of fishes came to the author’s attention in course 
of experiments performed in 1911 (Shelford and Allee). This work 
was followed by that of several students. The important papers by 
- Palitzsch and Sorensen—see bibliography in Clark (’20)—were over- 
_ looked by the writer until 1916. These important papers appear not to 
_ have been appreciated until 1915. The aid and advice of physical chemists 
was sought, but these investigators regarded differences in pH of less 
than unit value—e. g. between 8.0 and 9.01—of little importance, though 
they made important contributions to the subject. The turning-point of 
phenolphthalein was stated by Washburn (’15) to be pH 9.0; its turning- 
point in water analysis (faint pink lasting 3 minutes) is pH 8.0; all 
depends upon the method employed. 
The summary of Wells’ paper (’15) previously referred to, should 
read about as follows: 
1. Hydrogen ion concentration is an important factor in determin- 
ing the reaction and resistance of fishes. 
, 2. Most fresh-water fishes select slightly alkaline water in a gra- 
dient, but when offered a gradient from the turning-point of phenol- 
phthalein to higher hydroxyl ion concentrations (pH 8.0-9.0) the more 
alkaline end is selected. 
3. The optimum CO, varies from 0 (pH 8.0) for bluegills to 6 c.c. 
per liter in water with about 200 parts per million alkalinity (pH 7.4-7.6) 
for sunfishes and crappies. Optimum sulphuric acid was not determined 
because of carbonates in the distilled water used. 
4. The distribution of plankton in Wisconsin and New York lakes 
shows fewest animals in the stratum at the turning-point of phenol- 
phthalein (pH 8.0), suggesting a negative reaction to water with pH 8.0. 
: 5. No good results were obtained with once-distilled water because 
of carbonates present. 
The chief occasion for criticism of Wells’ paper is the confusion 
relative to neutrality. A repetition of his experiments, which was one of 
the purposes of the present paper, and a close study of his work shows 
that he regarded the turning-point of phenolphthalein (pH 8.0) as neutral, 
or “near enough” to it. The avoidance of pH 8.00 when this accom- 
_ panied higher alkalinity, as found by Wells, has been confirmed by Miss 
Hall in case of young whitefish. The table constructed by Wells showing 
the avoidance of pH 8.0 by plankton (New York and Wisconsin lakes) 
is correct in this respect, but the columns to the right and left of the 
phenolphthalein neutrality are by no means comparable in hydrogen ion 
concentration. The statements by Wells relative to CO,, together with 
those in the paper of 1911 by Shelford and Allee, are correct only for 
the “alkalinities” used, which varied around 100 p.p.m. of CaCO, or 
120 p. p. m. of HCO,. Several statements quoted from Wells (715) and 
