METHODS FOR THE MICROSCOPIC STUDY OF BACTERIA 195 



bacteria may be forced through filters and contaminate the filtrate 

 if the process is carried much beyond this interval. 



New, unused filters should be cleaned by running several liters of 

 clean water through them and they should invariably be tested before 

 use to guard against " pin-holes." 



After filtration the filter is sterilized to kill whatever bacteria have 

 contaminated it. Then the surface is thoroughly scrubbed with a 

 brush and 1 per cent, alkaline permanganate solution (potassium per- 

 manganate 10 grams, water 1000 c.c.) is run through to remove organic 

 matter. Five per cent, oxalic acid is then passed through to remove 



FIG. 91 FIG. 92 FIG. 93 FIG. 94 



FIGS. 91 to 94. Types of unglazed porcelain filters. (Park.) 



the permanganate solution and the acid finally removed by repeated 

 washings with water. If the filter becomes so clogged with organic 

 matter that it can no longer deliver a reasonable amount of filtrate, 

 the filter is placed in a muffle-furnace, gradually heated to about 

 250 C., and as gradually cooled. It is then cleaned as before with 

 permanganate solution, to remove the last traces of organic matter. 



Storage of Media. If media are not to be used at once it is necessary 

 to protect them from evaporation and contamination. Flasks of media 

 are preserved best by tying paper caps over the cotton plugs if the 

 period of storage does not exceed a few days, or by pouring melted 

 paraffin over the plugs if longer periods of storage are contemplated. 



