612 Stiles and j0rgensen.—Studies in Permeability , //. 
acid solution was actually o-ixo N, so that the acid used in the experiments 
had a concentration of o-oon N. This low concentration was used as it is 
unlikely to damage the plant-cells for some time ; a higher strength of 
acid is likely to be more dangerous in this regard. 
The experiments were carried out in stoppered bottles. In each bottle 
was placed ioo c.c. of acid and 20 discs of potato. A number of such 
bottles were placed in a thermostat kept at the desired temperature, and at 
Fig. t. This shows the form of hydrogen electrode used. The point electrode is shown at E. 
The liquid whose acidity is to be determined is placed in the larger vessel. Hydrogen is passed 
through the electrode vessel for a few seconds. By moving the plunger of the syringe backwards 
and forwards a few times while the gas is passing, the last traces of air are removed from the 
apparatus. The current of gas is stopped, and by pushing the plunger of the syringe in and out the 
electrode is wetted with the liquid. Finally the liquid is brought to such a level that the point just 
touches it. Equilibrium is very quickly attained with an electrode in good working condition. 
various intervals of time the bottles were removed, usually in duplicate, the 
acid poured off, and the concentration of the acid solution measured. 
In order to measure the acidity of solutions in this dilution, into 
which moreover various organic substances may have diffused out from the 
cell-tissue, the ordinary titration methods are useless. The measurement 
has therefore been made by means of the hydrogen electrode. 
When a metal is immersed in a solution of one of its salts, an electro¬ 
motive force is set up at the surface of contact of the metal and salt, and 
this E.M.F, is dependent upon the concentration of the metal ion in the 
