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equal to that of the aggregate length of all the water drops in E at any 

 one time. But the air escapes through H, under control of a pinchcock, 

 and the water is forced out through G. The waste water escapes through J. 



The flow of air through H must be so regulated that water is forced 

 out through G just a little faster than it enters from F. This provides for 

 an occasional release of surplus pressure by the escape of air through G 

 and prevents the filling of K with water, as will be the case if the air is 

 allowed to escape too fast through H. The only irregularity of flow is at 

 the time of the release of pressure through G ; but the air stream is seldom 

 interrupted for more than a few seconds, and by careful adjustment the 

 frequency of these interruptions may be reduced to a minimum. Perfect 

 adjustment would entirely eliminate these irregularities by allowing the 

 water to escape through G just as fast as it enters through F ; but, per- 

 fection being impossible, it is better to have the interruption occur as an 

 escape of air through G than of water through H. 



Theoretically the pressure of the air issuing from H, and consequently 

 the depth to which a solution can be aerated, is determined by the vertical 

 distance from the level of the water in K to the outlet of G. In practice, 

 however, the apparatus falls somewhat short of this, due to friction of 

 the air through H and the capillarity of the liquid to be aerated. The 

 density of the culture solution is, of course, a determining factor also. 



The efficiency of the apparatus depends lai-gely upon the nature of the 

 tube F. If it is of too small bore, the friction is too great ; and if it is 

 too large, the water has a tendency merely to run down the inside surface 

 and fails to carry any air with it. A very satisfactory size of tube is 

 one having an internal diameter of 2 to 4 mm. If a larger quantity of air 

 is needed at H, the pressure to remain the same, it is better to use two 

 tubes for F than to try to increase the capacity by substituting one larger 

 tube. If the pressure is to be increased and the amount of air to be de- 

 livered in a given time is to remain the same, G must be lengthened, and 

 this may necessitate the lengthening of F also, for F will carry air only 

 so long as the aggregate length of its water column is greater than that 

 in G. In adjusting the apparatus, glass or metal stopcocks have been 

 found more satisfactory where the flow of water is to be regulated, while 

 pinchcocks on pieces of rubber tubing have been found best for regulating 

 the stream of air. 



When well adjusted and in good working condition the apparatus is 

 economical. Tests on the one now in use have shown that it can be made 



