ed,
upon the withdrawal of which they again assume their solid condition.
Such media should be referred to as _liquefiable media_; in point of
fact, however, they are usually grouped together with the solid media.

NOTE.--It must here be stated that the designation 10 per
cent. gelatine or 2 per cent. agar refers only to the
quantity of those substances actually added in the process
of manufacture, and _not_ to the percentage of gelatine or
agar, as the case may be, present in the finished medium;
the explanation being that the commercial products employed
contain a large proportion of insoluble material which is
separated off by filtration during the preparation of the
liquefiable media.

Other media, again--e. g., potato, coagulated blood-serum,
etc.--cannot be again liquefied by physical means, and these are spoken
of as _solid_ media.

The following pages detail the method of preparing the various nutrient
media, in ordinary use (see also Chapter XI), those which are only
occasionally required for more highly specialised work are grouped
together in Chapter XII. It must be premised that scrupulous cleanliness
is to be observed with regard to all apparatus, vessels, funnels, etc.,
employed in the preparation of media; although in the preliminary stages
of the preparation of most media absolute sterility of the apparatus
used is not essential.

MEAT EXTRACT.

A watery solution of the extractives, etc., of lean meat (usually beef)
forms the basis of several nutrient media. This solution is termed "meat
extract" and it has been determined empirically that its preparation
shall be carried out by extracting half a kilo of moist meat with one
litre of water. For many purposes, however, it is more convenient to
have a more concentrated extract; one kilo of meat should therefore be
extracted with one litre of water, to form "Double Strength" meat
extract.

It was customary at one time, and is even now in some laboratories to
use either "shin of beef" or "beef-steak"--both contain muscle sugar
which often needs to be removed before the nutrient medium can be
completed. Heart muscle (bullock's heart or sheep's heart) is much to be
preferred and from the point of economy, ease and cleanliness of
manipulation, and extractive value, the imported frozen bullock's hearts
provide the best extract.

Meat extract (Fleischwasser) is prepared as follows:

1. Measure 1000 c.c. of