A filter of pyrex glass and stainless steel for use with Seitz filter-pads.

WE have designed a holder for use with Seitz EK bacterial filter-pads (diameter 3 cm.) in which the filtering liquids come in contact only with pyrex glass and a disc of stainless steel. This apparatus has the advantage that toxic metals (e.g. copper) cannot be dissolved by the filtering liquids as from the ordinary metal holder from which the plating (e.g. silver or rhodium) has been worn. Also the glass filter-tubes can be cleaned with chromic-sulphuric acid mixture; this is essential in much work in bacterial nutrition where traces of active material can produce growth. The design is shown in Fig. 1, drawn to scale. The pyrex glass filter-tubes A1 and A2 (made to our design by Messrs James A. Jobling, Wear Glass Works, Sunderland) are supplied with their flanges ground flat. A shallow recess is