BACTERIA DEVELOPING AT DIFFERENT TEMPERATURES 251 



oxygen being present in the winter, when the water is cold, than 

 in the summer, when the water is warm. The relationship between 

 these two factors is shown quite clearly in Table 16. A study of 

 the temperatures of the water, and of the amounts of dissolved oxygen 

 at times of high and low water, reveals the fact that the average 

 temperature was high, and the average amount of dissolved oxygen 

 was low, when the river was low, these factors decreasing and 

 increasing respectively as the volume of flow in the river increased. 

 The average results of eight years' determinations of the tempera- 

 ture and dissolved oxygen, arranged according to the volume of 

 water flowing in the river, are shown as follows in Table 16 : 



TABLE 16. 

 RELATION OF TEMPERATURE AND DISSOLVED OXYGEN TO VOLUME OF MERRIMACK RIVER. 



The effect of different amounts of dissolved oxygen in the water 

 is shown quite clearly in Table 17, in which the bacterial results 

 have been arranged according to the amount of oxygen present. The 

 numbers of bacteria and of B. coli were both at a maximum when 

 the dissolved oxygen in the water was between 0.50 and 0.75 parts 

 per 100,000. As the amount of oxygen increased or decreased 

 from these limits, the numbers of bacteria and B. coli decreased, 

 the numbers of B. coli decreasing much more rapidly with the increase 



TABLE 17. 



RELATION BETWEEN AMOUNT OF DISSOLVED OXYGEN AND THE BACTERIAL CONTENTS OF MERRIMACK 



RIVER WATER. 



in dissolved oxygen than did the bacteria. This is shown by the 

 bacteria-5. coli ratios in the last column of the table, the ratios 



