1900] MICROSCOPICAL JOURNAL. 31 



it was .012 gram per hour, and after 96 hours it was .002 

 gram per hour. This retardation of the rate of evapora- 

 tion appears to be due to a thickening of the gelatin at 

 the surface. If such is the case, the thickened gelatin at 

 the surface might materially reduce the supply of oxygen 

 of the submerged bacteria and prevent the development 

 of the less vigorous aerobic forms. Indeed, it was ob- 

 served that the plates in the desiccator and in the moist 

 chamber differed more in the number of submerged colo- 

 nies than in the number of surface colonies. 



. These facts suggested the possibility that the Petri dish 

 itself might serve to prevent the bacteria from getting a 

 sufficient supply of oxygen. The experiment was made, 

 therefore, of cultivating the bacteria in a Petri dish with 

 the ordinary cover replaced by a ground glass top that 

 made an absolutely air tight joint. A series of five cul- 

 tivations made in this way gave the number of bacteria 

 in a certain sample as 317 per cc, while a series of culti- 

 vations of the same water made in the usual manner gave 

 413 per cc. In the hermetically sealed dish, therefore, 

 only 77 per cent of the bacteria developed. The air of 

 these dishes was then collected over mercury, and the 

 amount of oxygen determined by absorbtion with pyro- 

 gallic acid. It was found that the air of the ordinary 

 dishes contained approximately 15 per cent of oxygen, 

 while the air of the sealed dishes contained only 5 per 

 cent. In other words, three-quarters of the original sup- 

 ply of oxygen in the sealed dishes had been used up by 

 the bacteria during 72 hours, while one-quarter of the or- 

 iginal supply had been used up in the ordinary dishes. It 

 was found also that the air in the sealed dishes contained 

 5 per cent of carbon dioxide, while the air in the ordinary 

 dishes contained but 2 per cent. 



As a matter of interest a series of cultures was made 

 in a jar filled with oxygen, and compared with cultures 

 of the same water made in the incubator. It was found 



