Stiles and J0rgensen.—Studies in Permeability. /. 363 
With alkalies a similar phenomenon is observed. Sodium hydrate in 
N N 
various concentrations from — to - has been used, and a fall in 
350 2000 
conductivity of the external solution has resulted, as in the case of acids. 
In order to obtain more definite information as to absorption of acid by 
plants we have measured the actual change in the concentration of the 
hydrogen ion by the use of the hydrogen electrode. It is well known that 
the contact E.M.F. between a metal and a solution of one of its salts is 
dependent upon the concentration of the ions of the metal in the solution, 
and the same applies to hydrogen and acids. A considerable time was 
spent in trials of numerous forms of hydrogen electrodes. Finally we have 
adopted the platinum-point form recommended by Michaelis ( 13 ) and 
Walpole ( 26 ). This has the advantage of enabling rapid work, and gives all 
the accuracy necessary for our purpose. 
N 
The E.M.F. of the hydrogen-electrode was measured against an — 
KC 1 calomel electrode, a solution of 3-5 N KC 1 being used as intermediate 
liquid. Kahlbaum’s potassium chloride with certificate of guarantee was 
used, and the mercurous chloride and mercury used for the calomel 
electrode were specially purified. 
By this method of investigation it was found that the hydrogen ion is 
indeed absorbed rapidly by the tissue. The electrical conductivities of the 
solutions were also taken, and from the two sets of results it becomes 
possible to form an idea of the quantity of electrolytes which have diffused 
out from the plant. 
An attempt was also made to determine the change in concentration of 
the chlorine ion by the use of the electrode chloride solution-calomel- 
mercury, but the results so obtained are not reliable, for the change in 
E.M.F. produced by addition of chlorine ions in such an electrode is due to 
a depression of the number of mercury ions in contact with mercury, and so 
anything which diffuses out of the plant and can cause a depression of the 
mercury ions will produce a change in the E.M.F. of the electrode. 
The following example will indicate the extent to which the hydrogen 
ion is absorbed by plants: 
N 
Pure HC1 was used with potato as in the preceding experiments. 
The conductivity and concentrations of hydrogen ions of the solutions were 
measured at various intervals of time. For each pair of measurements two 
sets of potato discs and solution were used, and conductivity and hydrogen 
ion concentration of both were measured. The following table gives the 
results obtained by this means. The results are illustrated graphically 
in Fig. 13. 
We do not propose to give here further data regarding the very impor- 
B b 2 
