BACTERIOLOGICAL TECHNIC. 49 



to filter. Both must be filtered hot, using hot-water funnels; or the ordinary 

 filtering device can be used by keeping the unfiltered portion hot and pouring 

 into the funnel from time to time. Cover funnel with filter paper to keep out 

 dust, and keep in the heat as much as possible. In so far as possible filter all 

 media through filter paper (one thickness, properly folded), but it is practi- 

 cally impossible (for reasons of time) to pass agar through filter paper. This 

 medium is usually filtered through cotton upon which a neatly folded and 

 perforated sheet of filter paper has been placed. Puncture the filter paper 

 several times with a small knife blade. Filtering through cotton is quick, 

 but the media are much less clear than when filtered through filter paper. 

 The filtering process may also be hastened by means of pressure (suction) ; 

 connect funnel with aspirator bottle and pump, but see to it that the connec- 

 tions with the hydrant are properly made and that the flow is properly reg- 

 ulated, in order to guard against any back pressure, which may cause the 

 receiver to fill with hydrant water. This accident is best avoided by 

 interpolating a flask or bottle. Agar may also be clarified by pre- 

 cipitation. Pour the hot agar into an ordinary percolator used by phar- 

 macists. The dirt particles and other impurities will gradually settle to the 

 bottom. When cool, take out the solid medium and cut away the lower 

 portion cantaining the sediment. 



C. Neutralization of Culture Media. As already stated, most bacteria 

 grow best in neutral or very slightly alkaline (to litmus) media, and since 

 most media are quite decidedly acid in reaction, it is desirable to alkalinize. 

 This is done by means of normal sodium hydroxide solution. In order to 

 understand the method of procedure clearly, it is necessary to make certain 

 explanations. 



A normal (N/i) solution of any substance contains as many grams per 

 liter of the substance as there are units in its molecular weight, if the substance 

 contains one atom of replaceable hydrogen. If it contains two atoms of 

 replaceable hydrogen, the number of grams used equals the molecular 

 weight divided by two, and so on. According to this, a normal solution of 

 sodium hydroxide contains 40 gm. of sodium hydroxide in a liter. 

 Exact normal solutions are, however, not prepared by weight. Crystallized 

 oxalic acid is used as the basis for making normal solutions. This 

 acid has a molecular weight (including a molecule of water of crystallization) 

 of 126, and, since it is dibasic, 63 gm. per liter are taken. Any normal 

 acid solution will exactly neutralize an equal volume of normal alkaline 

 solution. To make a normal sodium hydroxide solution, add about 41 gm. 

 of pure caustic soda to one liter of distilled water. Determine the amount 

 of this solution required to just neutralize i c.c. of normal oxalic acid 

 solution. This volume contains the quantity of sodium hydroxide which 

 should be present in i c.c. of normal solution, and from this we may 



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