1338 
If the current is off, the iron plunger in the evacuated glass 
vessel G is down and keeps the mercury in the horizontal side- 
tube, allowing the main current to pass between the sealed in 
platinum contacts. A rise of a fraction of a degree causes the 
large volume of mercury in the regulator C to make contact with 
the nickel spiral, starting the 8 volts current and thereby lifting 
the plunger out of the mercury in G. The running back of this 
mercury interrupts the main current and stops its heating effect 
very quickly, the heating wires being rapidly cooled, owing to 
their position in the open space between the table and the perforated 
bottom of the thermostat. Another advantage of this arrangement is 
the automatie stirring of the air in the thermostat by the jets of 
hot air rising through the holes in the bottom. 
The back and front walls are double glass windows; the 
latter can be lifted up, allowing the necessary operations to be 
performed in the thermostat. These take only a few minutes. The 
temperature in the inside is so quiekly restored after shutting the 
window, that the apparatus are practically maintained at the required 
temperature. 
As will be seen in Fig. 18, the potentiometer is connected by 
thin wires, passing through the walls of the thermostat, with the elec- 
trode inside, with the capillary electrometer, mounted on a box, 
which contains an accumulator for its small lamp, and on the other 
side with the working accumulator and the Weston cell, contained 
in the third box. 
As mentioned in part 5 of this paper, a direct estimation of the 
concentration of hydroxyl-ions, or of poy, in 8°/, phosphate solu- 
tions had become indispensable. 
The principle, on which these determinations were based, was 
the following: 
By saturating a blacked platinum electrode with oxygen, an OH 
electrode may be obtained in the same way as a H electrode is 
made with hydrogen. 
If z, be the potential of the calomel electrode with saturated KCl 
solution and aoy that of the OH electrode, the electromotive force, 
measured in the usual way at 27° is 
-y 
( 
Bret HON Te 1 0,095 log — , 
€ 
where C represents the concentration of OH, corresponding to the 
electrolytic solution pressure of the OH electrode, and c the OHion 
concentration. In such a cell the oxygen electrode is positive, the 
calomel electrode negative. 
