No. 453-] 



AERATING LIQUID SOLUTIOA 



lary siphon and that of the aerating tube, (2) the length of the 

 aerating tube, and (3) the depth of the sohition in the culture 

 jar. All that is necessary in order to increase the amount of 

 air, as compared with that of the solution in the tube, is to 

 increase either the diameter or length of the aerating tube or to 

 decrease the depth of the solution in the culture jar. 



If the diameter of the tube is increased and the flow kept 

 constant it is clear that more air must be taken in to fill the 

 increased space due to the enlargement. But as the diameter 

 increases the adhesive force between the walls of the tube and 

 the solution in it decreases, as compared with the cohesive force 

 of the solution, so that when a certain diameter is reached the 

 bubbles of air break through the columns of solution and thus 

 can no longer be forced to the bottom of the tube in the culture 

 jar. As a matter of fact, the maximum limit of diameter in the 

 case of water is practically about 4.5 mm. Further, if the rate 

 of flow in the aerating tube be in any way increased while that 

 in the siphon remains constant, more air will be required to keep 

 the tube full, just as in case of an increase in diameter. The 

 rate of flow in the tube, however, varies directly with the length 

 of the tube ; and since there is no theoretical limit to the possi- 

 ble length of the tube, the amount of air compared with that of 

 solution can be theoretically increased indefinitely. The depth 

 of the culture solution is really a function of the length of the 

 aerating tube. In order to overcome the pressure of the culture 

 Hquid and force the air out at the bottom of the aerating tube, 

 it is necessary to have a column of solution in the tube above 

 the level of the solution in the culture jar slightly greater than 

 the air space in the tube below that level. Consequently the 

 portion of the tube which is below the level of the solution in 

 the culture jar and enough of it above that level to contam as 

 much solution as there is air below, have nothing to do with 

 increase in acceleration due to gravitation, for they simply bal- 



: each other, and therefore these parts 



are not effective i 



increasing the amount of air introduced. This being true, it 

 will readily be seen that increasing the depth of the culture solu- 

 tion has the same effect as decreasing the length of the aeratmg 

 tube, and that therefore the deeper this solution, other thmgs 

 being equal, the less will be the air forced through. 



