212 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 
suitable electrode vessel, and connected by a narrow glass tube con- 
taining a suitable electrol\'le in solution. One electrode, of known 
potential, consists essentially of pure mercury overlaid with pure 
calomel and a solution of potassium chloride of known concentration. 
This may, for convenience, be a saturated solution, a y^ solution, or 
any other desired known amount. The second electrode consists 
in essence of a platinum plate coated with platinum black saturated 
with hydrogen, and partly immersed in the solution to be tested 
(which also contains hydrogen ions). By means of a suitable potenti- 
ometer the difference in potential, or electromotive force, developed 
between the two electrodes upon the passage of a suitable current 
from a battery, is measured. A relationship exists between the 
electrical potential of the gas chain, set up as described, and the 
hydrogen-ion concentration of the solution being tested. ^ From the 
hydrogen-ion concentration thereby deduced, the requisite amount 
of acid or alkali may be added to the original medium, and a second 
determination is then made to confirm the change in reaction. 
It is a well-recognized fact that the addition of standard (yjy) alkali 
to acid culture media, using a suitable indicator to detect the reac- 
tion change, does not give as sharp an end-point when the turning- 
point of the indicator is reached, as the same standard solution gives 
when added to a solution of a strong acid. This is due in part to the 
fact that the "weak" organic acids formed commonly in cultural 
media, as lactic or acetic acid, do not dissociate to any extent into 
hydrogen, and acet- or lact-ions, respectively. The phenomenon is 
also partly due to the presence in the medium of certain salts which 
act as sponges or buffers, absorbing the hydroxyl ions, but not react- 
ing with a sharply definite color change to an addition of alkali which 
would ordinarily be expected to be decisive. 
The explanation of this phenomenon- has its counterpart in the 
blood and tissues of the body which are kept at a strikingly uniform 
reaction in spite of the continuous production of carbon dioxide in 
the ordinary process of intracellular combustion, and the irregular 
introduction into the blood stream of the body of strong acids, as 
sulphuric acid, which arise from the transformation and absorption 
of digestive products in the intestinal canal. 
The blood plasma contains CO 2 and XaHCOs in such proportions 
that when the CO 2 content exceeds the NaHCOs content in the pro- 
portion of 1 part of CO2 to 21 parts of NaHCOs, the respiratory 
center is stimulated to increased activity and the excess of CO 2 is 
removed through the lungs. If, however, sulphuric acid is added to 
the plasma, the sulphuric acid, being the stronger, replaces CO 2 in 
the bicarbonate molecule, and sodium sulphate is produced, which 
1 Clark: The Determination of Hydrogen Ions, Baltimore, 1920. See Jones (Jour. 
Infec. Dis., 1919, 25, 262) for a simple apparatus for determining hydrogen-ion con- 
centration. 
- See Henderson (Das Gleichgewicht zwischen Basen und Sauren im Tierischen 
Organismus, Erg. d. Physiol., 1909, 8, 254, for full details. 
