STREAM POLLUTION 
411 
it must be added, as previously pointed out in the section on pH of natural water, 
that Brown and Jewell (1926) found fish living in a bog lake where the acidity of 
the water was pH 4.5 due to the action of carbon dioxide and organic acids leached 
from the surrounding bog vegetation. It is evident from all of the data in table 8 
that no matter what the diluent water lie, as far as hydrogen-ion concentration alone 
is concerned, the acidity near the magnitude of pH 4.0, regardless of the acid or acid- 
salt combinations producing this acidity, will be lethal for fresh-water fishes if that 
concentration of acidity be maintained. All acid concentrations more acid than 
pH 4.0 were lethal. In concentrations less acid than pH 4.0 the penetrative prop- 
erties and the lethality of the kation of the acid must also be considered, for it may be 
seen that at any given pH the relative lethality varies with the particular acid involved. 
On the basis of p. p. m. of acid present, disregarding the pH factor, the acids 
may be arranged in order of lethality as follows: Tannic acid, 10 p. p. m.; chromic 
acid, 100 p. p. m.; sulphuric acid, 130 p. p. m.; hydrochloric, 159 p. p. m.; benzoic, 
nitric, oxalic, and tartaric, each 200 p. p. m.; acetic acid, 348 p. p. m.; lactic, 430 
p. p. m.; and citric, 625 p. p. m., in the particular hard water tested. The exact 
p. p. m. values are not so significant, for they will vary to some extent with the 
hardness of the diluent water; but the group shows the general order of magnitude 
of the relative lethality for these common pollutant acids. 
Fishes are killed by acid wastes first through the precipitation and coagulation 
of the mucus on the gills and by the coagulation of the gill membranes themselves. 
If this coagulation of gills and gill secretions does not take place, the death of the 
fish is attributable to the lethal action of the kation of the acid. Combinations of 
these two actions are not uncommon, i. e., both precipitation and specific toxic 
action may contribute to the death of the fish. The precipitation of the mucus and 
of the proteins within the gill membrane cells themselves progresses rapidly when 
the relative acidity of the mixture is more acid than pH 4.5 because of the acidity 
itself, but this precipitation may be enhanced by the action of the kation of the acid. 
Tannic, chromic, and nitric acids all have marked affinity for living protoplasm, 
forming insoluble compounds with certain protein constituents of living tissue very 
promptly; in fact, all three of these acids are used as histological fixing agents to 
kill protoplasm. In the previous comparison on the basis of pH value, the minimal 
lethal concentration of these acids shows that tannic, chromic, and nitric killed gold- 
fish in solutions less acid than the lethal solutions of sulphuric, hydrochloric, citric, 
and lactic acids. Acetic acid has high penetrative properties and causes a swelling 
of tissues which is very destructive to the living cells, so that although this acid does 
not coagulate the gill membrane and gill mucus like hydrochloric acid, it does disrupt 
the cells of gill membrane with disastrous results. Consequently, it is not surprising 
to find that acetic acid kills goldfish at pH of 5.5. 
Reviewing all of the data on acid wastes, it seems that the truly acid effects 
must be limited largely to those acids which kill at a hydrogen-ion concentration 
more acid than pH 5.0; while in the cases of those acids killing at hydrogen-ion 
concentration less acid than pH 5.0, lethality factors other than hydrogen-ion con- 
centration play the major part. Of course, as the hydrogen-ion concentration of 
the surrounding medium in which the fishes and other aquatic animals are living 
becomes more acid than pH 7.2 to 7.4 (the normal values for most living cells), 
