of normal soda solution or 0.5 c.c.
of decinormal soda solution.

~Standardising Nutrient Agar and Gelatine.~--The method of standardising
agar and gelatine is precisely similar to that described under bouillon.

THE FILTRATION OF MEDIA.

~Fluid media~ are usually filtered through stout Swedish filter paper
(occasionally through a porcelain filter candle), and in order to
accelerate the rate of filtration the filter paper should be folded in
that form which is known as the "physiological filter," not in the
ordinary "quadrant" shape, as by this means a large surface is available
for filtration and a smaller area in contact with the glass funnel
supporting it.

To fold the filter proceed thus:

1. Take a circular piece of filter paper and fold it exactly through its
centre to form a semicircle (Fig. 100, a).

2. Fold the semicircle exactly in half to form a quadrant; make the
crease 2, distinct by running the thumbnail along it, then open the
filter out to a semicircle again.

3. Fold each end of the semicircle in to the centre and so form another
quadrant; smooth down the two new creases 3 and 3a, thus formed and
again open out to a semicircle.

4. The semicircle now appears as in figure 100, a, the dark lines
indicating the creases already formed.

5. Fold the point 1 over to the point 3, and 1a to 3a, to form the
creases 4 and 4a, indicated in the diagram by the light lines. Fold
point 1 over to 3a, and 1a to 3, to form the creases 5 and 5a.

[Illustration: FIG. 100.--Filter folding: a, Filter folded in half,
showing creases; b, appearance of filter on completion of folding;
c, filter opened out ready for use.]

6. Thus far the creases have all been made on the same side of the
paper. Now subdivide each of the eight sectors by a crease through its
centre on the opposite side of the paper, indicated by the faint broken
lines in the diagram. Fold up the filter gradually as each crease is
made, and when finished the filter has assumed the shape of a wedge, as
in figure 100, b.

When opened out the filter assumes the shape represented in figure 100,
c.

The folded filter is next placed inside a glass funnel supported on a
retort stand, and moistened with hot distilled water before the
filtration of the medium is commenced.

~Liquefiable solid media~ are filtered through a specially made filter
paper--"papier Chardin"--which is sold in boxes of twenty-five
ready-folded filters.

[Illustration: FIG.