86 PRACTICAL BACTERIOLOGY. 



tions air-tight, as they are never set aside for so lorig 

 a time as those in test-tubes. Should it, however, ap- 

 pear desirable to do so, especially when species that 

 grow very slowly are to be examined, then the edge of 

 the lower dish is carefully and quickly smeared over 

 with vaseline before the cover is put on. 



Cultivations in the hanging drop must be kept moist 

 by covering the edges of the cover-glass with vaseline 

 or liquid paraffin. With potato cultures it is best to 

 place a layer of sterilised blotting paper under the 

 slice of potato before it is cooked ; this paper be- 

 comes so saturated during the cooking, that it keeps 

 the culture moist for several days ; moreover, it can 

 always be wetted again with sterilised water if neces- 

 sary. A diflSculty very frequently presents itself, 

 which is not very easily overcome. Evaporation takes 

 place so rapidly from the large surface, that water con- 

 denses on the lid. By degrees this collects in drops, 

 which soon become so large as to fall down on the layer 

 of agar-agar, possibly completely spoiling the culture. 

 This inconvenience may be got over to some extent 

 , by turning the dish over, so that the smaller surface, 

 covered with agar-agar, forms the lid. If then the water 

 vapour condenses below, a thing which never occurs 

 so quickly, it will, just as it is about to turn into drops 

 of water, be re- absorbed by the agar-agar. In many 

 cases also it answers to sterilise pieces of blotting paper 

 which exactly fit the lids, with the dishes, and then 



