ring the tissue of sufficient
consistency to admit of thin slices or "sections" being cut from it.
This is effected by passing the tissue successively through alcohols of
gradually increasing strength: 30 per cent. alcohol, 50 per cent.
alcohol, 75 per cent. alcohol, 90 per cent. alcohol, absolute alcohol.

In both these processes a large excess of fluid should always be used.

FREEZING METHOD.

1. ~Fixation.~ Place the pieces of tissue in a wide-mouthed glass bottle
and fill with absolute alcohol. Allow the tissues to remain therein for
twenty-four hours.

2. ~Hardening.~ Remove the alcohol (no longer absolute, as it has taken up
water from the tissues) from the bottle and replace it with fresh
absolute alcohol. Allow the tissues to remain therein for twenty-four
hours.

[Illustration: FIG. 71.--Washing tissues.]

NOTE.--If not needed for cutting immediately, the hardened
tissues can be stored in 75 per cent. alcohol.

3. Remove the alcohol from the tissues by soaking in water from one to
two hours. Remove the stopper from the bottle; rest a glass funnel in
the open mouth and place under a tap of running water. The water of
course, overflows, but the tissues remain in the bottle (Fig. 71).

4. Impregnate the tissues with mucilage for twelve to twenty-four hours,
according to size. Transfer the pieces of tissue to a bottle containing
sterilised gum mixture.

~Formula.~--

Gum arabic 5 grammes
Saccharose 1 gramme
Boric acid 1 gramme
Water 100 c.c.

5. Place the tissue on the plate of a freezing microtome (Cathcart's is
perhaps the best form), cover and surround with fresh gum mixture;
freeze with ether, or for preference, carbon dioxide, and cut sections.

6. Float the sections off the knife into a glass dish containing tepid
water and allow them to remain therein for about an hour to dissolve out
the gum.

(If not required at once, store in 90 per cent. alcohol.)

7. Transfer to a glass capsule containing the selected staining fluid,
by means of a section lifter.

8. Transfer the sections in turn to a capsule containing absolute
alcohol (to dehydrate) and to one containing xylol or oil of cloves (to
clear).

9. Mount in xylol balsam.

_Alternative Rapid Method._--

1. Cut very small blocks of the tissue.

2. Fix in formalin 10 per cent. aqueous solution (fixation
fluid No. 7, page 82) for 24 hours.

3. Transfer block to p