82 A MANUAL OF BACTERIOLOGY 



posited on the plate and examined, and those in which 

 only a single organism can be found are noted and the plate 

 is then incubated so that colonies may form, from which 

 sub-cultures may be prepared. 



Esmarctis roll cultures. Another modification of the 

 plate-culture method is known as Esmarch's roll culture. 

 For this purpose large test-tubes (" boiling tubes "), at 

 least an inch in diameter and six inches long, are sterilised 

 and plugged with cotton- wool. The sterile melted gelatin, 

 about 10 c.c., is poured in and inoculated, the wool plug 

 replaced, and the tube held in the horizontal position and 

 rotated under a stream of cold water, or in warm weather 

 on a block of ice, until the gelatin has set. In this way 

 the gelatin forms a thin film over the inside of the tube, 

 but a little practice is required to get it evenly distributed. 

 The colonies then develop in the film of gelatin, which is 

 quite analogous to a film in a Petri dish. 



Anaerobic plate cultivations are sometimes required. 

 The plate culture after preparation as described above, 

 using a deep Petri dish, is inverted, and some alkaline 

 pyrogallol is placed in the lid ; this absorbs the oxygen 

 within the dish. The preparation must be kept under 

 observation for the next hour or so, and more alkaline 

 pyrogallol is added from time to time to compensate for 

 the rise of fluid within the dish until absorption of the 

 oxygen from the contained air is complete. 



McLeod has devised a useful porcelain dish for con- 

 taining the alkaline pyrogallol over which the Petri dish 

 is inverted, the joint being made air-tight with plasticine. 



In Botkin's method a bell-jar standing in a glass dish 

 is made use of. The Petri dishes are placed on a support 

 within the bell- jar, and mercury or oil is poured into the 

 glass dish. By means of a piece of bent glass tubing a 

 stream of hydrogen is passed into the bell- jar under its 

 rini so as to displace the air, which bubbles out through 



