4IO APPENDIX. 



Another flask may be inserted, and so on until the whole of the fluid is 

 filtered. At the end of the process, such fluid as remains in the filter is 

 withdrawn by means of a sterilized pipette, and may be used along with 

 the other, though it is better to use a fresh filter for each flask, as then the 

 indiarubber communication between the flask and the filter may be clamped 

 and the flask removed. The fluid within the flasks so removed may 

 be kept sterile for a considerable length of time. Another very con- 

 venient filter is Kitasato's, which consists of a large thistle shaped funnel 

 attached by an indiarubber connection to a piece of very thick pipe stem 

 of unglazed porcelain, which is well-plugged with baked porcelain at the 

 bottom. This pipe stem passes down through the indiarubber cork into a 

 bottle which serves as a receiver. From the side of the neck of the receiver 

 a lateral tube is given off, to which the exhaust apparatus may be attached. 

 The apparatus is thoroughly sterilized as above. The fluid to be filtered is 

 placed in the funnel, the exhaust is applied and the filtrate passes down 

 into the bottle. Here also it is well to place a small plug of cotton-wadding 

 in the lateral tube. In all cases where the water exhaust pump is used it 

 is well to have a bottle intervening between the receiver and the pump, so 

 that should there be any backward flow of water it may pass into this flask 

 and not into the receiver. This filter requires a somewhat stronger exhaust 

 than the Chamberland pattern, and an ordinary aspirating siphoh bottle 

 is not sufficient to obtain the required suction. Various modifications 

 of this apparatus will at once suggest themselves to an ingenious worker, 

 but one that may sometimes be used, especially where the culture 

 fluids have to be kept at a low temperature, is that recommended by Miquel. 

 It consists of a flask with a long wide neck ; at some distance above the 

 bulb (about halfway) this neck is considerably constricted, and between 

 the constriction and the bulb is a long narrow lateral tube which is pointed 

 somewhat do\vnwards. To prepare this as a filter, the tube above the 

 constriction is packed with asbestos, above this is poured some thoroughly 

 dried and sterilized plaster of Paris made into a cream-like paste with 

 boiled distilled water, this should be nearly an inch in thickness. The 

 plaster is allowed to "set," and the filter is ready for use. A small 

 quantity of water is put into the flask and thoroughly boiled, the steam 

 escaping from the lateral tube. As soon as the water is boiled away, but 

 while the flask is still full of steam, the lateral tube is sealed. The whole 

 apparatus is now thoroughly sterile : an indiarubber bung with a funnel pass- 

 ing through the centre is fitted into the tube above the plaster of Paris ; 

 into this the fluid to be filtered is poured. As the air in the flask cools 

 (and eventually ice may be packed round the flask) the fluid is drawn 

 through the plaster of Paris ; this flask can seldom be more than about half 

 filled in this manner. After the indiarubber cork has been removed, the 

 flask may be kept in ice, the contents remaining sterile for an indefinite 

 period. 



Hanging Drop Cultures. 



The best form of moist chamber for making drop cultures is an ordinary 

 slide into which is cut a deep round groove which surrounds a central pillar 

 or disc of glass which has been ground and polished so as to be slightly 

 below the level of the remainder of the slide. This, after careful steriliza- 

 tion, is used as follows : A small drop of the fluid is placed upon the lower 

 central pillar (a small portion of the fluid may also be allowed to run into 



