IOWA ACADEMY OF SCIENCES. 



71 



The chemical work began in 1898 and continued from 

 the seventh to twenty-sixth of October of that year. Com- 

 mencing in 1899 samples were taken from January 10 

 to October 1, 1901. During this period samples of the 

 manhole or raw sewage, tank and effluent were taken 

 weekly and analyzed as soon as possible on reaching the 

 laboratory. After October 1, 1901, samples were taken 

 each month only. 



