12 MICRO-ORGANISMS AND FERMENTATION. 



particles floating in a liquid are secretions or bacteria. Sub- 

 stances are added to the liquid which favour the growth of 

 bacteria, and by lengthened observation of the behaviour of 

 the particles it may be determined whether they propagate 

 or not. 



Amongst various kinds of moist chambers, that of Bottcher 

 (Fig. 4), which now finds extensive use, may be mentioned. 

 It consists of a glass ring fastened to a common object glass, 

 and upon this a cover-glass is cemented with vaseline. The 

 cover-glass carries on its under side a freely suspended drop 

 containing the object to be examined. A few drops of water 

 are placed on the floor of the chamber to prevent evaporation 

 of the suspended drop. If the cover-glass has been completely 

 freed from grease by cleaning with acid and ether, the drop 

 will spread out to a thin layer, so that it may be observed 

 under a strong power, and with a short focal distance. As 

 the drop hangs freely, it is possible to lift the cover-glass 



Fig. 4. 



without disturbing the growth, if a sample is to be removed. 

 If the cells are to be fixed, a little gelatine may be mixed with 

 the liquid before sterilisation, as suggested by Brefeld. In 

 his detailed researches on micro-organisms, Brefeld used this 

 and other kinds of moist chambers, which are illustrated and 

 described in Vol. IV. of his Botanischen Untersuchungen uber 

 Schimmelpilze (Leipsic, 1881). In order to secure the presence 

 of a single cell in the drop, he diluted the infected liquid 

 until this proved to be the case. If the organism demands 

 a full supply of air to reach complete development, a stream 

 of moist air may be passed through two tubes fixed in the 

 sides of the glass ring. 



By the help of such apparatus definite conclusions can be 

 arrived at regarding the nature of the growth of micro- 

 organisms, and this knowledge is essential for accurate work 

 with mass cultures. To prepare the latter, the small pure 



