METHODS FOR MICROSCOPIC STUDY OF BACTERIA 223 
made to his work for particulars which are important but })eyond the 
scope of this book. , , i , 
New, unusued filters of the Berkefeld and Pasteur types should be 
cleaned by running several liters of clean water through them and they 
should inVariablv be tested before use to guard against ''pin-holes." 
Fig. 21. — Autoclave. (Park.) 
After filtration the filter is sterilized to kill whatever bacteria have 
contaminated it. Then the surface is thoroughl\- scrubbed with a 
brush, and 1 per cent alkaline permanganate solution (potassuim 
permanganate 10 gm., water 1000 cc.) is run through to remove organic 
matter. Five per cent oxalic acid is then passed through to remove the 
