The application of sintered or fritted glass filters to bacteriological work was reported by Morton and Czarnetzky in 1937. The sintered glass filters available at that time were made of Jena laboratory glass. That glass is no longer available and it is very doubtful if filters of Jena glass will be available after the war. The constant interest in and the use of sintered glass filters for certain bacteriological work prompted an investigation to determine if filters manufactured from "PYREX"

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... and Chemical glass N 774 in this country are efficient in retaining bacteria. Such filters were stated to have a maximum pore diameter of 1.3 to 1.9 microns and were designated UF (ultra-fine) by the manufacturer.' Filters of the Buchner type containing a 35 mm filtering area (40 mm disc) filtered 100 ml of distilled water in 7 to 9 minutes under a pressure of 660 to 690 mm of mercury. Fifty ml of a 1:10 dilution in broth of a 24 hour old broth culture of Serratia marcescens were filtered in 7 to 35 minutes by the same filters under similar conditions. All filtrates remained sterile during an observation period of one week. To determine the length of time required for the test organism to penetrate the "PYREX" sintered disc, a filter was assembled so that sterile broth was in contact with both sides of the disc. The broth on one side of the disc was inoculated with Serratia marcescens and the broth on the other side of the disc subcultured periodically. The subcultures made after 7' and 17 hours remained sterile, whereas those made at 24 hours sometimes showed the presence of the test organism. The filters are cleaned as previously described and assembled in the conventional manner (Morton, 1938, Morton and Czarnetzky, 1937) . The safest way to insure that all the acid cleaning fluid has been washed out of the filtering disc is to test the pH of the wash water. Browne (1942) reported that unbuffered solutions became more alkaline when filtered through Seitz filters. That point has been verified with distilled water. The pH of distilled water is not changed when it is filtered through a clean "PYREX" sintered disc. REFERENCES BROWNE, H. H. 1942 Changes in reaction caused by filtration through Seitz filters.