526 A. FRANKLIN SHULL 



equaled that dissolved in water in which green organisms were 

 kept as food. A number of tests to determine this relation were 

 made, some of them closely resembling the experiments, others 

 differing more or less, but bearing directly on the oxygen content 

 of treated water. The oxygen in solution was measured by the 

 Winkler method. 



In this paper all measurements of oxygen are given in number 

 of cubic centimeters per liter of water. This quantity is com- 

 puted from the tests by means of the formula 



55.825 b n 

 o = 



t V 



in vvhich 'b' is the number of cubic centimeters of potassium 

 bichrcmate solution used in standardizing the sodium thiosul- 

 phate; 't' is the number of cubic centimeters of sodium thio- 

 sulphate required in titration against 'b' cc. of potassium bi- 

 chromate; V' is the volume in cubic centimeters of the water 

 sample tested; and 'n' is the number of cubic centimeters of the 

 thiosulphate required for the water sample. In the tests here 

 described 'b' was always 25, the other factors being variable. 



Direct solution of oxygen. — In table 3 are described the tests to 

 determine the amounts of oxygen dissolved in water when the 

 water is agitated in contact with an atmosphere containing an 

 excess of oxygen. Test No. 1, consisting of seven parts, was most 

 nearly like one of the oxygen experiments, and was furthermore 

 most complete. This test indicates that the water used in one of 

 the rotifer experiments contained 6.81 cc. of oxj'gen per liter at 

 the outset (a) . After it was shaken with an atmosphere of which 

 40 per cent was oxygen, it contained 11.33 cc. of oxygen per liter 

 (b). When the food (manure scum) was added, the water con- 

 tained only 7.58 cc. of oxygen (c). After standing a day under a 

 bell jar, in an atmosphere of which 40 per cent was oxygen, the 

 oxygen content of the water had risen to 8.35 cc. per liter (d). 

 Had the dish been kept in air instead of under a bell jar, the 

 oxygen content would have fallen to 5.43 cc. per liter (e). 



