AGf~ICULTURAL RESEARCH, PUSA.

CULTIVATION OF MICRO-ORGANISMS 89
cent. of glycerol to ordinary agar. The former is used chiefly
for deep stab cultures of anaerobes (vide p. 13G).
STANDARDISATION OF MEDIA
'Vhile many bacteria show vigorous growth within
a fairly wide range of reaction as regards acidity or
alkalinity, there are others "which require the reaction
of the medium to be adjusted within narrow limits
before multiplication takes place. Moreover, all organisms
have a particular reaction at which the growth
is optimal.
In order, therefore, to secure the best growth,
particularly of the highly parasitic organisms, it is
necessary that the adjustment of the reaction should
be made as accurately as possible.
T\,{O methods are given here-the one in whieh
the aeidity or nlkalillity is expressed in terms of t1w
absolute acidity which depcnds 011 the hydrogen-ion
concentration, and the other in relation to the neutral
point of an indicator (phellol phthalein). The former
method is the more satisfactory.
STANDARDISATION ACCORDING TO HYDROGEN - ION
CONCENTRATION. - The true acidity of any fluid
depends on the number of dissociated hydrogen-ions
present, and the reaction of the medium is dependent
on, and measured by, the hydrogen-ion concentration.
The greater the concentration, the morc acid the
medium.
The standard of hydrogen-ion concentration is Nil
acid (e.g. NIl HeI), which, if fully dissociated, contains
1 gram of hydrogen-ions per litre. Then N/10 is only
1/.10, or 10-1, of this concentration, N/IO,OOO is 10- 4 ,
and N/l,OOO,OOO is 10- 6 , of the standard hydrogen-ioll
concentration.
The hydrogen-ion concentration is expressed in
the form of the "hydrogen exponent" designated
pH, which is the logarithm of the concentration as
compared with the standard, with the negative sign
omitted.