and add it to the equivalent of the n/1 NaOH solution
previously run in. Take the total as the correct estimation.

_Method of Expressing the Reaction._--

The reaction or _titre_ of meat extract, medium, or any solution
estimated in the foregoing manner, is most conveniently expressed by
indicating the number of cubic centimetres of normal alkali (or normal
acid) that would be required to render _one litre_ of the solution
exactly neutral to phenolphthalein.

[Illustration: FIG. 99.--Stock bottle for dekanormal soda solution.]

The sign + (plus) is prefixed to this number if the original solution
reacts acid, and the sign - (minus) if it reacts alkaline.

For example, "meat extract + 10," indicates a sample of meat extract
which reacts acid to phenolphthalein, and would require the addition of
10 c.c. of _normal_ NaOH per litre, to neutralise it.

NOTE.--Such a solution would probably react alkaline to
litmus.

Conversely, if as the result of our titration experiments we find that
25 c.c. of meat extract require the addition of 5 c.c. n/10 NaOH to
neutralise, then 1000 c.c. of meat extract will require the addition of
200 c.c. n/10 NaOH = 20 c.c. n/1 NaOH.

And this last figure, 20, preceded by the sign + (i. e., +20), to
signify that it is acid, indicates the reaction of the meat extract.

NOTE.--The standard soda solutions should be prepared by
accurate measuring operations, controlled by titrations,
from a stock solution of 10N NaOH, which should be very
carefully standardised. If a large supply is made or the
consumption is small this stock solution must be kept in an
aspirator bottle to which air can only gain access after it
has been dried and rendered free from CO_{2}. This may be
done by first leading it over H_{2}SO_{4} and soda lime, or
soda lime alone, by some such arrangement as is shown in
figure 99, which also shows a constant burette arrangement
for the delivery of small measured quantities of the
dekanormal soda solution.

STANDARDISATION OF MEDIA.

Differences in the reaction of the medium in which it is grown will
provoke not only differences in the rate of growth of any given
bacterium, but also well-marked differences in its cultural and
morphological characters; and nearly every organism will be found to
affect a definite "optimum reaction"--a point to be carefully determined
for each. For most bacteria, howeve