METHODS FOR MICROSCOPIC STUDY OF BACTERIA 211 
volume of water, filtered through a cold wet filter paper to remove 
fat, and made up to a volume of a liter. The sokition contains some 
meat extractives, including a relatively large proportion of xanthin 
bases and a very considerable amount of salts, particularly sodium 
chloride. Little or no muscle-sugar is present. It is distinctly 
inferior to meat infusion, however, as a basis for cultural media, 
especially for the more delicate pathogenic organisms. 
The Reaction of Media.— The extent, and in no inconsiderable degree 
the nature, of bacterial growth is influenced by the reaction of their 
nutritive environment. There is an optimum reaction for all microbes, 
which cannot be exceeded materially either toward the alkaline or acid 
side of this point without creating an environmental factor fletrimental 
to development. Those microorganisms which are parasitic or path- 
ogenic for man grow best in media ha^'ing an initial reaction very 
slightly on the alkaline side of absolute neutrality.^ 
The true reaction of media depends in the final analysis upon the 
concentration of hydrogen (and hydroxyl) ions which they contain, 
and it follows necessarily that a proper adjustment of the hydrogen- 
ion concentration— the reaction of media, in other words— is a pre- 
requisite to successful bacterial cultivation. Media containing peptone, 
meat extractives or meat solutions are usually too acid for bacterial 
cultivation until the acidity is reduced to the vanishing point, or even 
carried to the faintly alkaline side of neutrality. This is commonly 
accomplished by adding the requisite amount of alkali. The process 
involves two distinct steps: The determination of the degree of 
acidity or deviation from the point of neutrality, and the rectification 
of the reaction by the addition of the proper amount of alkali. 
The determination of the reaction of media is made by one of three 
principal methods: The determination of the hydrogen-ion concen- 
tration by the gas chain potentiometer apparatus, the colorimetric 
determination of the hydrogen-ion concentration by the color com- 
parison method with suitable indicators and standard hydrogen-ion 
solutions, or by the method of titration to the end-point of a particular 
indicator with standard alkali (or acid) solution. 
The gas-chain method is the most accurate, but the most time- 
consuming and cumbersome, especially when a considerable number 
of determinations are to be made. The apparatus required is also 
very expensive. 
The principle involved is to measure the flifference in potential 
or electromotive force developed between two electrodes, each in a 
1 The condition of absolute neutrality is practically realized in the purest water, which 
contains in addition to H2O, the equivalent of one-ten-millionth normal acid, as hydro- 
gen ions, and the equivalent of one-ten-millionth normal alkali, as hydroxyl ions, in 
1 liter. In other words, 10 million liters of the purest water would contain 1 gram of 
ionized hydrogen and 1 gram of ionized hydroxyl. The hydrogen-ion concentration of 
of water, therefore, is 1 X 10-', or pH = 7, and the hydroxyl-ion concentration of 
water is 1 X 10-', or pOH = 7. It is customary to neglect the OH ion, and to write 
the reaction in terms of ionized hydrogen only, thus: pH 5 corresponds to a _ ^ 
acid and pH 9 to a i^Jooff alkali solution respectively. 
