288 



Earle B. Phelps 



EXPERIMENT 7. 

 Started March 5, 1906. 



Temperature, 20°-24°. 



DISCUSSION OF RESULTS. 



Effect of organic matter in tap water. — In comparing the results 

 of these experiments, the basis of comparison will be the number of 

 surviving organisms in each case, expressed as percentage of the 

 initial number. Other bases of comparison suggest themselves, but 

 the one selected appears on the whole to be the most logical. In 

 this way the effect of the copper upon those few comparatively re- 

 sistant organisms, always found in experiments of this nature, is 

 given predominant importance, while the effect upon that large per- 

 centage of organisms which is killed in all the experiments has com- 

 paratively little weight. 



Such a comparison of the average results of Experiments i and 

 3, best water, with those of Experiment 2, tap water, gives the 

 following figures: 



It is quite evident that the organic matter present in the tap water 

 inhibits the toxic action of the copper, but the significant fact here is 

 that there is no definite point in the series at which the addition of 

 more copper to the tap water will produce a normal killing effect 

 such as is produced in pure water. The effect of the organic matter 



