30. 
III. When is the pollution most concentrated? 
In fresh water, pollution will, as a rule, be most concentrated during 
seasons of drought or in extreme low-water in winter; but to this rule 
there are many exceptions. Pollutions which float will do most damage 
during storms or high winds. This source of danger is greatest in the 
sea. 
Effect of Winter Ice—Ice in winter prevents aeration and hinders 
circulation, and seems to have been responsible in Illinois for important 
losses of fish due to pollution (Hansen and Hilscher, ’16, 16a). 
Effect of Temperature —Many poisons are more toxic at high tem- 
perature than at low (Warren, ’00). Results soon to be published by 
Powers confirm Warren’s conclusions. Experiments should generally be 
carried on when the water in question is at the usual seasonal tempera- 
ture, though some tests should always be made at its maximum tempera- 
ture. Some of the relative-resistance work on adult fishes at different 
seasons of the year may have been vitiated to some extent by too little 
regard to the temperature factor, thus yielding somewhat exaggerated 
differences of resistance in the comparisons made (Wells, ’16). Temper- 
ature has direct effects also (Wells, 714). 
Effect of Oxygen—An abundant supply of dissolved oxygen re- 
duces the toxicity of some substances—of CO,, for example (Wells, 713; 
Shelford, ’18b). 
IV. What is the toxicity of untreated polluting effluents; of each 
residual of processes of partial recovery; or of treatment by additions to 
the effinent? : 
One essential in dealing with pollution problems lies in the deter- 
mination of the toxicity of the various constituents of the polluting sub- 
stance and of the wastes or residues resulting either from operations— 
present or prospective—for the recovery of useful substances or from 
methods of treating the effluent. This is imperative, for partial-recovery 
methods may give rise to substances as residues or secondary by-products 
which are as toxic as the original waste, or even more so. As a Case in 
point I may mention a personal study of a large series of representative 
constituents of gas wastes which I began in 1914. This investigation 
showed that all the essential components are so highly toxic that no 
method of partial recovery can be relied upon to protect fishes. The least 
suspected substances, some rated as insoluble, were among the most toxic 
—for example, CO, benzene, and naphthalene (Shelford, 17; Wells, 18). 
The proposed plan of treating the sewage of Boston and Chicago 
and other cities of more than 50,000 inhabitants by the Miles acid process 
(Weston, 16) in order to recover grease, ammonia, and glycerine, and 
to leave a strongly acid effluent which, in the case of Boston and other 
coast towns, would provide a sterile medium for oyster-beds, is a matter 
M 
a) are fatal, or 
essentially so, to the eggs of such marine animals as have been tested with 
demanding very careful investigation, since acids ( 
