ing position in the inspissator at 75 deg. C.
for one hour.

9. Place the tubes for forty-eight hours in the incubator at 37 deg. C.,
and eliminate any contaminated tubes.

To prevent drying, 0.5 c.c. glycerine bouillon (see page 209) may be
added to each tube between steps 8 and 9.

10. Cap those tubes of media which remain sterile with india-rubber caps
and store for future use.

~Potato.~--

1. Choose fairly large potatoes, wash them well, and scrub the peel with
a stiff nail-brush.

2. Peel and take out the eyes.

3. Remove cylinders from the longest diameter of each potato by means of
an apple-corer or a large cork-borer (i. e., one of about 1.4 cm.
diameter).

The reaction of the fresh potato is strongly acid to phenolphthalein.
If, therefore, the potatoes are required to approximate +10, as for the
cultivation of some of the vibrios, the cylinders should be soaked in a
1 per cent. solution of sodium carbonate for thirty minutes.

4. Cut each cylinder obliquely from end to end, forming two wedge-shaped
portions.

5. Place a small piece of sterilised cotton-wool, moistened with sterile
water, at the bottom of a sterile test-tube; insert the potato wedge
into the tube so that its base rests upon the cotton-wool. Now plug the
tube with cotton-wool (Fig. 111).

6. Sterilise in the steamer at 100 deg. C. for twenty minutes on each of
_five_ consecutive days.

[Illustration: FIG. 111.--Potato tube.]

NOTE.--The cork borer reserved for cutting the potato
cylinders should be silver electro-plated both inside and
out, and the knife used for dividing the cylinders should be
of silver or silver plated. When these precautions are
adopted the potato wedges will retain their white color and
will not show the discoloration so often observed when steel
instruments are employed.

~Beer Wort.~--Wort is chiefly used as a medium for the cultivation of
yeasts, moulds, etc., both in its fluid form and also when made solid by
the addition of gelatine or agar. The wort is prepared as follows:

1. Weigh out 250 grammes crushed malt and place in a 2-litre flask.

2. Add 1000 c.c. distilled water, heated to 70 deg. C., and close the
flask with a rubber stopper.

3. Place the flask in a water-bath regulated to 60 deg. C. and allow
the maceration to continue for one hour.

4. Strain through butter muslin into a clean flask and heat in the
steamer for thirty minutes.

5. Filter th