Stream Pollution 49 



conditions, and these will include, of course, the regulation of the 

 volume of flow, the degree of pollution of the waste, the temperature 

 of the water, the abundance of oxygen, carbon dioxide, etc. It is a 

 well established fact that the temperature of the water affects the 

 sensitiveness of fish and conditions which may be injurious at one 

 time are not serious in their effects at another. Furthermore, Wells 

 has shown most distinctly that an abundance of oxygen tends to 

 reduce the toxicity of various other substances in solution, especially 

 of carbon dioxide. 



Effects of Prolonged Influence 



In order to reach a more accurate measure of the injurious 

 character of polluted waters, one would have to take into account 

 the effects of the prolonged influences of a waste on the fish. We 

 recognize generally from our study of the human race that dele- 

 terious influences are frequently cumulative in effect and that even 

 small quantities of poisonous materials, acting upon the organism 

 over a long period of time, will produce serious and even fatal 

 results. In a similar way extremely diluted chemical substances, 

 which might seem in an experiment of brief duration to have no 

 effect whatever on fish living in them, will, undoubtedly, in given 

 cases be found to affect adversely the condition of fish subject to 

 their influences continuously for a longer period of time. This 

 question is one on which little or no experimentation has been made. 

 We are forced to speak of the conditions in terms of analogy, basing 

 our conclusions upon the results that are well known in the records 

 of human disease and have been confirmed in individual cases 

 among other animals. The kinds of materials which exercise such 

 deleterious influences, the amounts adequate to produce the results 

 feared, and the other conditions under which they must work are, 

 however, not fully known. 



Limiting Factors 



A considerable number of other factors should also be taken into 

 account in connection with the minnow test. Unless care be ex- 

 ercised in determining the time and other conditions under which 

 the test is made, one may very easily get a set of conditions that is 

 calculated to prevent rather than to demonstrate the actual effect 

 of the pollution. The influence of a waste discharged into a stream 

 must be measured by its effect under most unfavorable conditions, 

 for those are the times in which the fish life will be injured or 



