658 THE AMERICAX XATl'RALISr. [Vol. XXXVIIL 



The purpose of the constriction in the aerating tube is merely 

 to break up the cohimn of sokition in the tube. If this were not 

 thus broken up it woukl accumukite until it became slightly 

 greater than the air spaces in the tube below the level of the 

 culture fluid and then would suddenly flow down, forcing the air 

 out with a rush, after which it would again accumulate, flow 

 down, etc. This is just what happens if the constriction is not 

 small enough in comparison with the size of the capillary siphon. 

 A few exijerimental trials, however, will be sufficient to enable 

 one to decide on the proper relative sizes of the two. No defi- 

 nite instruction can be given with reference to this since the 

 diameter of the siphon must vary with the diameter and length 

 of the aerating tube and with the dejith, specific gravity and 

 viscosity of the culture solution. In general the constriction 

 should be somewhat smaller than the siphon. 



In some forms of apparatus^ the lower end of the aerating 

 tube is bent on itself so as to project upwards in the culture 

 solution and a small piece of sponge is then pressed rather 

 tightly into this bent free end, or the tube may be drawn out 

 into a capillary ending. It is claimed that the sponge, espe- 

 cially, breaks up the air into very small l^ubblcs. This is un- 

 doubtedly true, but it has been ^"ound that in the ai)paratus 

 represented in Figures i and 2 the pressure recjuired to force 

 the air through the si)onge is so great that the value of this 

 adjunct is questionable. 



The most efficient and simplest method discoveretl to break 

 up the air bubbles is as follows: Cut off the lower end of the 

 aerating tube square, grind it quite flat, and let it rest on a firm 

 piece of rubber, leather, or wood on the surface of which a con- 

 siderable number of radiating grooves has been made. The 

 piece of rubber rests on the bottom of the culture jar and the 

 tube IS placed over the point whence the grooves radiate. The 

 grooves in any event must be narrow. Their depth should depend 

 upon the weight of the aerating tube. If the tube is he:ny it 

 will sink into the surface upon which it rests, and therefore 

 under such conditions the gro.n-es must be deeper than if the 



