4 LABORATORY MANUAL OF MICROBIOLOGY 



of normal sodium hydroxide required to make the reaction 

 pH 7.0. Check the results by repeating titrations. All attempts 

 to adjust the culture to an exact pH are unnecessary since the 

 changes in reaction due to heat, glassware, etc., will be found 

 greater than a pH 0.1. 



Filtration. Funnel Filter. — A cotton filter is prepared as 

 follows: In the base of a large funnel place a small amount of 

 clean excelsior. In place of the excelsior a small spiral of copper 

 wire or iron-wire netting may be used. On top of this put two 

 or three layers of absorbent cotton. Split a piece of absorbent 

 cotton, somewhat larger than the top of the funnel, horizontally 

 into two layers of equal thickness. Place one layer of cotton 

 above the other, so that the fibers are at right angles and wash the 

 cotton filter with boiling water. Pour the medium, slowly at 

 first, on to the filter. (In order to avoid breaking the filter use 

 a glass rod to direct the fluid to the center of the filter). When 

 the filtrate begins to come through the cotton, fill the funnel. 

 If the first filtrate is not clear, the turbid liquid should be refil- 

 tered through the same cotton. 



If several liters of media are prepared the suction method given 

 below will be found convenient. 



Suction Filter. — Prepare an absorbent cotton filter on the top 

 of a heavy walled glass bottle; milk bottles are satisfactory. 

 Place a layer of cheesecloth on the top of the bottle, then one or 

 two layers of absorbent cotton and cover with another layer of 

 cheesecloth. Be sure that the filter is tied tightly around the 

 neck of the bottle. Now invert bottle in a saucepan containing 

 the medium. It is sometimes necessary to use a weight to hold 

 down the empty bottle. Place in the autoclave and heat to 10 

 or 15 pounds pressure for 20 minutes. Allow to cool slowly. 

 The heat will exhaust the bottles and as it cools the medium is 

 drawn up through the filter pads. Since the vacuum in the 

 bottles is never complete, the bottles are usually found to be 

 about two-thirds full after filtration. 



Solid Media. — For the preparation of solid media, agar, gelatin, 

 or silica gel are commonly employed. Some of the differences 

 between these substances are shown in the following table: 



