CULTURES IN SOLID MEDIA. 73 



bered to correspond with the tubes. The plates used by Koch are 

 from eight to ten centimetres wide and ten to twelve centimetres 

 long. They must be carefully cleaned and sterilized in the hot-air 

 oven, at 150 C., for two hours. They may be wrapped in paper be- 

 fore sterilization, or placed in a metal box especially made for the 

 purpose. In order that the liquid gelatin may be evenly distributed 

 upon the plate the apparatus shown in Fig. 44 is used. This con- 

 sists of a glass plate, g, supported by a tripod having adjustable feet. 

 By means of the spirit level / the glass plate is adjusted to a hori- 

 zontal position. A sterilized glass plate is placed in the glass tray, 

 shown in the figure, and the gelatin from one of the tubes is care- 

 fully poured upon it and distributed upon its surface with a steril- 

 ized glass rod, care being taken not to bring it too near the edge of 

 the plate. The glass tray in then covered until the gelatin has 

 cooled sufficiently to become solid, after which plate No. 1 is re- 

 moved and plates Nos. 2 and 3 are made in the same way. In 



Fio.44. 



order to save time it is customary to fill the glass tray shown in the 

 figure with ice water, to place a second glass support upon it, and 

 upon this the sterilized glass plate upon which the liquid gelatin is 

 poured. This is protected by a glass cover, as before, until the gela- 

 tin becomes solid. 



The three plates, prepared as directed, are put aside in a glass 

 jar of the form shown in Fig. 44, one being supported above the 

 pther by a bench of sheet zinc or glass. 



Petri's Dishes. A modification of the plate method of Koch, 

 which has some advantages, consists in the use of three small glass 

 dishes of the same form as the larger one used by Koch to contain 

 the plates. These dishes of Petri are about ten to twelve centime- 

 tres in diameter and one to 1.5 centimetres high, the cover being of 

 the same form as the dish into which the gelatin is poured. These 

 dishes take less room in the incubating oven than the larger glass 

 jar used in the plate method, and they do not require the use of a 

 levelling apparatus. The colonies also may be examined and 

 counted, if desired, without removing the cover, and consequently 



