Stiles and Jergensen.—Studies in Permeability. II. 613 
solution of the salt. The same is true also for an electrode of hydrogen 
immersed in an acid, i. e. a solution containing hydrogen ions. 
As hydrogen electrode we used a modification of that suggested 
by Walpole (Bioch. Journ. 1913). The electrode vessel consisted of a small 
piece of glass tubing closed at one end with a rubber stopper. Through 
this stopper passed a glass tube with the electrode of platinized platinum 
wire sealed into the lower end, and a piece of capillary glass tubing con¬ 
nected to a T-piece. The other two ways of the T-piece were connected 
one to a small glass syringe, the other to an apparatus for generating 
hydrogen. This was carefully purified before passing into the electrode 
vessel. 
The electrode vessel could be placed in a larger vessel containing 
the liquid whose acidity was to be determined (Fig. 1). 
The electrode was charged in 
the usual manner as described by 
Walpole. 
The hydrogen electrode was com- 
N 
bined with an — KCl-Calomel elec- 
10 
trode, a 3*5 N solution of KC 1 
being used as intermediate liquid. 
Kahlbaum’s pure potassium chloride 
with certificate of guarantee was used 
and the mercury and mercurous 
chloride were carefully purified. The 
form of the calomel electrode is shown 
in the accompanying sketch (Fig. 2). 
The glass vessel itself is filled above 
N 
the mercury and calomel with — 
KC 1 saturated with calomel, and this solution occupies the tube between 
the vessel itself and the three-way tap. The intermediate liquid of 
3*5 N KC 1 occupies the rest of the tube, and can be run out by 
turning the tap, or renewed from the same funnel. The tap is ungreased 
and kept closed, there always being enough liquid held by capillarity 
to ensure sufficient conduction. 
The electromotive forces manifested by this combination were com¬ 
pared with a Weston Normal Cell manufactured by the Cambridge 
Scientific Instrument Company. The standard cell and the combination 
of calomel electrode and hydrogen electrode were compared with a 
2-volt accumulator by Poggendorfs method. A capillary electrometer 
(enclosed pattern) was first used as a null instrument, but this was after¬ 
wards replaced by a delicate moving coil galvanometer manufactured by 
Fig. 2. The diagram shows the form of 
Calomel electrode used. The bent part at A 
actually lies in a plane at right angles to the 
rest of the apparatus. The platinum point 
p dips into a mercury cup. 
