231 



throughout the year, and it may be that some of the seasonal 

 fluctuations in the chemical substances which have been dis- 

 cussed in the preceding pages, and some of those in the plank- 

 ton also, depend in some measure upon this changing effect of 

 temperature upon the sewage. 



The following table, taken from Jordan ('00), gives the 

 seasonal changes in numbers of colonies of bacteria from May 

 Bacteriological Examination of Illinois River at Havana. 



(I) June 21. (2) June 27. (3) July 6. (4) July 18. (5) July 25. (6) Aug. 8. (7) 

 Aug. 22. (8) Aug. 39. (9) Sept. 5. (10) Sept. 12. (11) Sept. 18. (12) Sept. 26. (13) 

 Oct. 3. (14) Oct. 10. (15) Oct. 31. (16) Nov. 7. (17) Nov. 30. (18) Dec. 5. 



30, 1899, till the end of the year at Havana, and at Pekin, 

 thirty-two miles above. The decline from the larger numbers 

 in June to a fairly well-maintained minimum during midsum- 

 mer at Havana is very evident, as is also the rise as the tem- 

 perature lowers in the autumn. Both the period of time and the 



