798 METHODS OF INVESTIGATING BACTERIA. 



plates of about 8 to 12 cm. in length, and 6 to 8 cm. in 

 breadth. A number of these are sterilised at 180 C., 

 and kept in a covered vessel ; the uppermost plate is 

 always taken, and the gelatine is placed on its lower 

 surface. We usually employ three plates, of which each 

 is laid on a large sheet of glass, which is kept level. If 

 the room is warm it is well to place a vessel contain- 

 ing cold water or ice beneath this glass plate. A mix- 

 ture of nutrient jelly and bacteria is then poured out on 

 its surface. This mixture is prepared in the following 

 manner: we take three test tubes each containing 8 to 10 

 ccm. of nutrient jelly, and the gelatine in each is 

 liquefied by dipping them in water at 40 C. A small 

 quantity of the material to be investigated is then intro- 

 duced into the first glass and slowly but carefully mixed 

 with the jelly ; from this mixture three to five loopfuls 

 are introduced into the second glass ; this is again 

 thoroughly shaken up, and from this glass three to five 

 loopfuls are introduced into the third. The contents of 

 these three glasses are then poured out on separate glass 

 plates, the gelatine being equally divided over the plate 

 by means of a previously heated glass rod. A strip 

 about 1^ cm. in breadth is left free all round the 

 margin ; on these free places sterilised glass supports 

 are placed and fixed with a few drops of gelatine, and 

 thus several plates may be piled one above the other. 

 The plates are kept covered with a bell -jar till the gela- 

 tine is quite firm; after ten to fifteen minutes they can be 

 placed in a glass vessel and set on one side. The vessel 

 and its cover are lined with moistened filter paper, and 

 sterilised in the steaming apparatus. These vessels 

 may be placed in incubators kept at a temperature of 

 about 22 C., and are examined every twelve or twenty- 

 four hours, at first without removing the cover, and later 

 by examining the plates under the microscope at a mag- 

 nification of 80 to 100 diameters. 



By employing three degrees of dilution as above 

 described, we almost always find that one of the plates 

 is good, i.e., it contains a few isolated colonies which 

 can be counted, and on the other hand not too few 



