18 MODERN DAIRY PRACTICE. 



paper, layers of sand, and so on, all prepared especially for 

 the purpose. These filters retain all solid particles found 

 in the liquids, and consequently also the bacteria. This 

 method of sterilization is not, however, applied as often as 

 the one previously given. 



Methods of Examination. The culture of the bacteria 

 takes place in the laboratory in sterilized glass vessels of 

 different shapes. Ordinary test-tubes or glass flasks are 

 most commonly used; in either case the mouth of the ves- 

 sel is closed by a sterilized cotton plug. This allows the air 

 to pass in and out, but prevents bacteria from gaining 

 access to the vessel from the outside. Sterilized gelatine 

 plates, properly protected against infection from the bac- 

 teria of the air, are also used. 



In the bacteriological examination of a liquid substance 

 this is first examined through the microscope ; the different 

 kinds of bacteria in the substance are then isolated. This 

 is done according to the method invented by Koch : one 

 or two drops of the liquid are introduced into a test-tube 

 containing some gelatine which has previously been lique- 

 fied by careful heating. The sample is shaken carefully 

 and rapidly, so that the gelatine cannot congeal, and is 

 then poured on a glass plate protected from infection of 

 bacteria and allowed to solidify. By the thorough shaking 

 the individual bacteria in the drops of the liquid are scat- 

 tered in the gelatine, so that on the congelation of the 

 latter they are fixed at different places in the same. If the 

 plate be now kept under favorable conditions of tempera- 

 ture, light, and moisture, each one of the single bacteria 

 will multiply and form colonies of varying sizes, as a rule 

 visible to the naked eye (see Fig. 18). 



If the sample does not contain such an abundance of 



