38 THE AMERICAN MONTHLY [Feb 



tliat the average number of colonies that developed in the 

 oxygen was 154 per cc, while in the incubator it \vasl37. 

 The growth in oxygen also took place at a much more 

 rapid rate. 



Finally, one important fact was noted in connection 

 with the cultures made in the jars that illustrates the im- 

 portant effect of oxygen. In all the jars the five Petri 

 dishes were ])iled one on top of the other, and it was found 

 that almost invariably the plate liighest in the jar gave 

 the largest count,and that the lowest plate gave the small- 

 est count. This was particularly striking in the case of 

 the jar filled with oxygen, where the upper plate contain- 

 ed 198 colonies per cc, while the Iowrt plate contained 138. 

 These differences between the upper and lower plates of 

 the jars were greater than the differences observed be- 

 tween plates kept in the incubator. Liquefaction pro- 

 ceeded much more rapidly in the upper plates of the jars. 

 These phenomena seemed to be due to an accumulation of 

 carbon dioxide at the bottom of the jars, where either by 

 its own properties or by exclusion of oxygen, it retarded 

 the growth of the bacteria. 



These facts tended to show that the supply of oxygen 

 has a marked effect upon the growth of water bacteria on 

 the gelatin plate, and that whatever tends to reduce the 

 supply of oxygen, whether it be evaporation or an air- 

 tight Petri dish or lack of sufficient ventilation, will tend 

 also to reduce the number of bacteria. The practical in- 

 ference from this is that in order to obtain the greatest 

 possible development of water bacteria on the gelatin 

 [)late, a ventilated dish should l)e used, and the cultures 

 should be incubated in an 'atmosphere saturated with 

 moisture. 



It has been found that 'satisfactory ventilation of the 

 Petri dishes may be obtained by grinding several small 

 notches in the edge of the lower plate. If greater ven- 

 tilation is required, it may be obtained by extending the 



