380 
These studies have been made by the author largely with a view to 
ascertaining whether or not hydrogen ion determinations give indica- 
tions of value in the field of animal ecology. The author’s answet is in 
the affirmative. It is accordingly the purpose of this paper to show (1) 
that hydrogen ion concentration is one of the factors governing the 
movement and distribution of fishes, its importance on the whole probably 
equaling that of well-known factors; (2) that breeding places of fish 
are characterized by differences in hydrogen ion concentration; (3) that 
hydrogen ion concentration has marked influence on the survival of 
fishes under adverse conditions. 
METHODS OF DETERMINING H Ion CoNcENTRATION (pH) 
The methods in common use are the colorimetric and the electro- 
metric. The former is based on the colors assumed by solutions of 
various dyes in the presence of various hydrogen ion concentrations. 
The latter is based on measurement of the differences of the electric po- 
tential between the solution to be tested and a hydrogen electrode. In 
this process it is necessary to bubble hydrogen through the sample, and 
as this withdraws CO, it is not practicable as a field method. It is de- 
sirable to dip the apparatus into the mud or water and measure while 
in situ. This might be possible except for the additional fact that the 
pressure of the hydrogen (gas) must be maintained at one atmosphere, 
or corrected to this pressure, which renders the use of the electrometric 
method impracticable in the field. Colorimetric standards may be color 
charts for rough work or for work on soils—those published in Clark’s 
book (’20) may be purchased separately—but for use in fresh water 
greater accuracy is necessary. For this purpose “buffer” solutions of 
known hydrogen ion concentration are placed in hard glass tubes, a 
standard amount of indicator added, and the tubes sealed up and used 
for comparison with water samples with the same amount and kind of 
indicator added. Several firms make such sets of standards, but the 
range at present is limited, and others must be added for some waters. 
These additional buffers may be made by a competent chemist. Standard- 
ization of buffers electrometrically is quite essential; also the purchased 
sets must be checked electrometrically at the beginning and end of each 
series of readings. 
