112 



LIQUID CULTIVATING MEDIA. 



Lister's Flask. 



62. A convenient method of storage for sterile liquids consists in 

 making use of Listers flasks (Fig. 57). They consist of spherical 



Fig. 57. — Lister's Flask. 

 Bohemian glass flasks with two necks — a vertical and a lateral one ; 

 the former is straight, and is plugged in the usual way with sterile 

 cotton wool (§ 41, p. 64) ; the latter consists of a bent tube, piercing 

 the side of the flask, and ending with a constricted neck in a tapering 

 nozzle. This lateral entrance is protected by a mass of sterile cotton 

 wool enclosed in a piece of gauze, and secured to the constricted 

 neck of the tube by a piece of twine or an elastic ring. These flasks, 

 which contain about half a litre each, are filled only so full as to 

 avoid the risk of their contents reaching either of the plugs during the 

 process of ebullition, in which case the clearness of the liquid might 

 be lost by the abstraction of dust or fibrils from the cotton wool of 

 which the plugs are made. When it has been charged with a quantity 

 of stock liquid, the Lister's flask is sterilised by subjecting it to 

 heat, that of the steam steriliser (Fig. 14) being preferred, from its 

 preventing evaporation during the sterilising process. The stock 

 liquid can then be kept for any length of time without becoming 

 contaminated. When it is desired to abstract some of the contents 

 of the flask, the sterile pad is removed from the bent lateral neck, and 

 as much liquid as is required is allowed to flow from the lateral 



