34 THE AMERICAN MONTHLY [Feb 



the greatest moment, there is such a lack of uniformity in 

 the details of the process as ordinarily conducted, that 

 the determinations of different observers are seldom com- 

 parable unless the methods of procedure are fully de- 

 scribed. A standard method of procedure is urgently 

 needed, but cannot be secured until the various factors 

 that influence the result have been analyzed and theirmagr- 

 nitude determined. In the course of a series of experi- 

 ments conducted with this object in view, it was observed 

 that the amount of moisture in the atmosphere of the in- 

 cubator exercised an important influence upon the number 

 of bacteria that developed. The results of these obser- 

 vations are summarized in this paper. 



Before the use of the Petri dish it was customary to 

 pour the mixed gelatin and water upon a cold glass plate, 

 where it was allowed to spread out and harden. The 

 plate was then covered with a bell-jar aud put in the in- 

 cubator. In order to prevent the gelatin from drying, 

 moist filter paper was put at the bottom of the bell-jar. 

 Thus the bacteria developed in a moist atmosphere. With 

 the advent of the Petri dish the matter of moisture seems 

 to have been lost sight of. This dish was provided with 

 a tight fitting cover, and this was supposed to prevent 

 evaporation from the gelatin. It is a fact, however, that 

 the Petri dishes now on the market are not tight, and that 

 often the covers fit the plates very badly. Furthermore, 

 in laboratories where many dishes are in daily use, it is a 

 common thing for dishes aud covers to be mismated, aud 

 often there is no attempt to mate them. The result is 

 that an appreciable evaporation from the gelatin does take 

 place, and that the amount of evaporation varies with 

 diS'erent plates and different atmospheric conditions. 



The effect which this uncontrolled factor exercises upon 

 the development of bacteria is shown by the following ex- 

 periments. Several series of cultures were submitted to 

 varying conditions of moisture, with other conditions re- 



