METHODS OF INVESTIGATING BACTERIA. 795 



placing a small drop of the nutrient solution on the 

 centre of a sterilised cover glass; the latter is then 

 seized with heated forceps, turned over, and placed over 

 the hollow in a sterilised excavated slide. Before doing 

 this it is well to place a layer of vaseline around the 

 cavity in the slide, and the cover glass is pressed on this 

 layer, and thus air is excluded (see fig. 141 ; a, the 

 hanging drop ; 6, the vaseline layer, which must not 

 touch the border of the drop). The development of the 

 bacteria in the drop can then he watched with high 

 powers of the microscope, either by the use of a warm 

 slide, or by Watson Cheyne's method (see p. 344). 



Access of air is necessary for the development of many 

 fungi, and the arrangement just described does not 

 permit it. In these cases Prazmowski has devised an 

 arrangement by which a small channel leads from the 

 moist chamber to the outer air, and is not closed by the 

 vaseline. These moist chambers do not permit a long 

 observation, because the closure is imperfect, and im- 

 purities, more especially mould fungi, are generally 

 found after a few days. 



The glass cells employed by von Recklinghausen and Glass cells. 

 Brefeld are more perfect arrangements. Brefeld's ap- 

 paratus, which has been specially constructed for the 

 cultivation of mould fungi and for their observation 

 with high powers, consists of a narrow glass tube which 

 dilates in the middle and forms a flattened sphere. The 

 walls of the chamber are only of the thickness of cover 

 glasses, and are so flat that an uniformly thin layer of 

 fluid may be readily obtained in their interior. These 

 vessels are thoroughly cleaned and freed from fat by 

 immersion in ether and subsequently in boiling water, 

 and then the fluid containing the fungus to be examined 

 is sucked in, so that the inner wall of the chamber is 

 only covered by a thin layer, and thus an individual 

 spore can be watched for days with a high power. 



Where larger cultivations are required we employ Advantages of 

 either fluid or solid nutrient substrata. The latter are 

 most suitable for obtaining and maintaining a pure culti- 

 vation, and they also form the best means for isolating 



