36 THE AMERICAN MONTHLY [Feb 



tween 65 per cent and 80 per cent, the average being 75 

 per cent. In a third series the atmosphere of the incu- 

 bator was kept moist by means of jars of water, and the 

 average humidity was 95 per cent. In a fourth Series 

 the humidity was similarly kept at 98 per cent of satura- 

 tion. The relation between the humidity and the devel- 

 opment of bacteria is shown by the following comparison 

 of the Series: Humidity, 60, 75, 95, 98 per cent while 

 the number of bacteria that developed in the incubator 

 was 75, 82, 98, or 97 per cent of the number that developed 

 in the moist chamber. 



The amount of evaporation from gelatin under the con- 

 ditions described above was determined by weighing the 

 dishes with their contents before and after incubation. It 

 was found that in a saturated atmosphere the evaporation 

 was inappreciable ; in the closed chamber it was from 1 

 to 2 percent of the weight of the mixed water and gel- 

 atin during 72 hours ; in the ice chest it was about 3 per 

 cent ; in the incubator it was from 3 per cent to 5 per 

 cent; and in the desiccator it was from 10 per cent to 15 

 per cent. The amount of evaporation varied greatly with 

 different plates submitted to the same conditions. 



This loss of water by evaporation is sufficient to cause 

 important changes in the composition of the culture media 

 during incubation, and this may be sufficient to affect the 

 development of the bacteria, as these organisms are very 

 susceptible to slight changes in environment. But the 

 chief reason why increased evaporation is accompanied by 

 a decrease in the percentage of bacteria that develop 

 seems to be connected with the supply of oxygen. By 

 successive weighing of the. same plates it was found that 

 the rate of evaporation was not constant, but decreased 

 rapidly. For example, during the first hour of incuba- 

 tion the evaporation from a series of plates proceeded at 

 the rate of .068 gram per hour; after 18 hours the rate 

 of evaporation was .031 gram per hour; after 42 hours 



