454 PROCEEDINGS OF THE AMERICAN ACADEMY. 



sary, the burette could be made to deliver its reagent under the layer of 

 kerosene, by means of tubing of rubber and glass. But without this 

 additional precaution consistent results were easily obtained and the 

 acidity of different cultures or of the same culture from day to day could 

 be compared. Of course carbonic acid was not the only acid which w as 

 estimated in the above process. The end reaction took place when the 

 first trace of normal carbonate or of excess of sodic hydroxide was 

 present. 



By means of this process the following observations were made. If 

 separate hay and leaf cultures be set with a salt solution as previously 

 described, the acidity of 5 cc. at the beginning may be zero or perhaps 

 equivalent to 0.1 cc. of 0.01 normal sodic hydroxide. After one day an 

 acidity of several tenths will have been reached by both cultures, but the 

 hay culture has the greater acidity. After two to three days the acidity of 

 5 cc. of the hay cultures was usually equivalent to approximately 0.7 cc. 

 of 0.01 normal sodic hydroxide. After the third day there was no im- 

 13ortant increase of acidity unless too much fermentable matter had been 

 added. Observation of the Stentors originally placed in these cultures 

 for seed showed that when the acidity exceeded an equivalency of 1 cc. 

 of 0.01 normal sodic hydroxide they became pale, rested upon the bottom 

 of the jar, and diminished in numbers. From the bottom of the hay 

 culture their disintegrating forms could be taken and examined. A 

 further examination of successful and unsuccessful cultures showed that 

 the former were either neutral or had an acidity of a few tenths only, 

 whereas the latter were nearly always characterized by a considerable 

 degree of acidity. The fact that heating any of the culture liquids ujj to 

 the boiling point greatly diminishes their acidity, often indeed producing 

 a condition that gives an akaline reaction with phenolthalein, points to 

 the presence of COo, produced by fermentation. As is well known, 

 organic acids also are developed in this process, but in smaller propor- 

 tions. Soon after the condition of maximum acidity is reached, a covering 

 of Stentors appears over the surface of the culture. In a successful 

 culture the increase in Stentors, Pararaaecia, and some other Protozoa 

 may have already reached a great abundance before this condition is 

 attained. Multiplication of Stentors was never observed in an actively 

 fermenting liquid. Observation both of natural and experimental cul- 

 tures shows that Stentor is an animal that has adjusted itself to such 

 conditions as prevail in a late stage of a fermenting liquid. To raise 

 Stentors in a salt solution, it is necessary to reduce to the minimum the 

 intensity of the fermentation inherent in the food supply. This fermeu- 



