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MICRO-ORGANISMS AND FERMENTATION. 



about 30 mm. diameter. This is fastened to a glass-ring, 

 which again is cemented to a thicker glass, thus forming the 

 moist chamber previously described (Fig. 2), which is adapted 

 to the purpose, and carries a layer of solid gelatine on the 

 inside of its upper surface. The essential point in HANSEN'S 

 method is, that the leading principle " the starting-point of a 

 pure culture must be a single cell " is consistently carried out, 

 which is not the case in KOCH'S method. The germs must be 

 so sparsely distributed that comparatively few are present in 

 the gelatine layer ; the chamber is then either allowed to 

 remain under the microscope, in order that the multiplication 

 of the germs may be directly followed, or the positions of 

 well isolated germs are marked, either by dividing the glass- 

 cover into small squares or by means of the object marker, 



FIG. 9. Moist chamber with edged squares. 



and the apparatus is placed in the incubator, or at the 

 temperature of the room, until the colonies are completely 

 grown. In the author's laboratory moist chambers like 

 that represented in Fig. 9 are used, the cover-glass 

 being etched with 16 squares and figures. The situation 

 of the cells is then marked on a sketch plan which shows 

 all the etched figures and squares. On one cover-glass there 

 may be 50 to 60 well isolated germs. When the colonies are 

 completely developed, they are transferred to flasks by means 

 of a small piece of platinum or copper wire, which has been 

 previously ignited and cooled. During this transference the 

 culture is momentarily in the air, and is here exposed to 

 contamination. But the danger of contamination at this, 

 the single weak point, is reduced to an insignificant 

 minimum, and disappears if the operation is performed in a 



