. Carnegie Institution of Washington publication. 44 BACTERIA IN RELATION TO PLANT of the cylindrical glass vessel. When sterilized and ready for use, select a piece of rubber cloth 10 or 15 cm. in diameter, cut a small slit in its center and draw it over the nipple of the bougie to protect the cotton from accidental wetting and the filtered fluid from consequent possible contamination. Now pour the fluid into the bougie (if one with a large neck has been selected this will not be difficult, especially if a small funnel is used and this is kept from close contact on one side by mean
. Carnegie Institution of Washington publication. 44 BACTERIA IN RELATION TO PLANT of the cylindrical glass vessel. When sterilized and ready for use, select a piece of rubber cloth 10 or 15 cm. in diameter, cut a small slit in its center and draw it over the nipple of the bougie to protect the cotton from accidental wetting and the filtered fluid from consequent possible contamination. Now pour the fluid into the bougie (if one with a large neck has been selected this will not be difficult, especially if a small funnel is used and this is kept from close contact on one side by means of a small wire, sliver, or bit of paper), and connect the nipple with the outflow-tube of the compressed-air pipe by means of an extra-thick rubber tube, which should be securely wired at each end, and turn on the compressed air cautiously. Fluids which are not colloidal usually filter very readily with a pressure of 15 or 20 pounds per square inch. The filtering should always be done slowly with a mini- mum pressure in order to avoid the possibility of forcing small organisms through the walls of the filter. With heavy pressure this sometimes occurs when no cracks are detectable in the bougie. When the desired quantity of fluid has been filtered (fig. 40) cut off the air-blast, disconnect the tube, tilt the cylinder as much as possible, remove the bougie, and substitute the sterile cotton plug. The fluid should now be transferred immediately, in 5 or 10 cc. portions, to sterile cotton-plugged test-tubes by means of sterile pipettes. The removal of the bougie and the transfer of the fluid should be done in clean still air, under a hood or in a special culture-room. The tubes should not be used for several days, i. e., time should be given for contaminations to show themselves, but if proper care has been exercised there should be very few contaminations or none at all. A pressure much greater than 20 pounds per square inch may be obtained by means of steam-pumps or by use of
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