32 



MICRO-ORGANISMS IN WATER 



or others gaining access during the operation, and in 

 all experiments it is necessary to pour such plates as a 



Glass plate 



Bell-glass 



Thick plate of 

 glass 



Levelling screw 

 Levelling stand 



FIG. 6. LEVELLING APPARATUS FOR MAKING PLATE-CULTIVATIONS. 



control for comparison with those which have been 

 purposely infected. 



The gelatine-plate method as applied to waters will be 

 described later in detail, but it will be convenient here 

 to give an account of how these plate-cultures are em- 

 ployed for the isolation of particular micro-organisms. 



For instance, supposing that we take some bouillon 

 which has been either imperfectly sterilised or purposely 

 exposed to contamination, we should probably find that 

 under the microscope a confused mass of forms would 

 be visible, and to separate out some of these different 

 varieties we abstract a small quantity of the liquid by 

 means of a sterilised instrument. For this purpose a 

 piece of platinum-wire about J T of an inch in thickness 

 and about three inches in length is commonly used, one 

 end of which is fused into a thin glass rod by melting 

 the latter in a bunsen-name, inserting the wire, and then 

 allowing the junction to cool slowly. The free end of 

 the wire is then twisted into a small oval loop, which 

 will contain quite a sufficient quantity of the liquid, and 

 the wire being readily sterilised by heating immediately 

 before use in the bunsen-flame, lends itself particularly 



