156 NUTRIENT MEDIA 



liver very small quantities ; but if the calculated amount 

 runs into thousandth parts of a cubic centimetre, these 

 are replaced by corresponding quantities of normal or 

 even decinormal soda. 



In the above example it is necessary to add 11.65 

 c.c. normal NaOH or its equivalent, 1.165 c - c - deka- 

 normal NaOH. The first being too bulky a quantity, 

 and the second inconveniently small for exact measure- 

 ment, the total weight of soda is obtained by substi- 

 tuting 1. 1 6 c.c. dekanormal soda solution, and either 

 0.05 c.c. of normal soda solution or 0.5 c.c. of deci- 

 normal soda solution. 



Standardising Nutrient Agar and Gelatine. The 

 method of standardising agar and gelatine is precisely 

 similar to that described under bouillon. 



THE FILTRATION OF MEDIA. 



. Fluid media are usually filtered through stout Swed- 

 ish filter paper (occasionally through a porcelain filter 

 candle) , and in order to accelerate the rate of filtration 

 the filter paper should be folded in that form which 

 is known as the "physiological filter," not in the ordi- 

 nary "quadrant" shape, as by this means a large 

 surface is available for filtration and a smaller area in 

 contact with the glass funnel supporting it. 

 To fold the filter proceed thus : 



1 . Take a circular piece of filter paper and fold it ex- 

 actly through its centre to form a semicircle (Fig. ioo,a). 



2. Fold the semicircle exactly in half to form a quad- 

 rant ; make the crease 2, distinct by running the thumb- 

 nail along it, then open the filter out to a semicircle 

 again. 



3. Fold each end of the semicircle in to the centre 

 and so form another quadrant; smooth down the two 

 new creases 3 and 3 a, thus formed and again open out 

 to a semicircle. 



