4 Proceedings of Royal Society of Edinburgh. [sess. 
electrode dissolves ; and, owing to the variable amount which 
dissolves, the potential between metal and electrolyte is thus a 
variable quantity. 
The silver used was in the form of stout wire and was pure. 
Two silver rods which had received different treatment were 
inserted in silver nitrate solution, and this cell was then inserted 
along with a Weston cell in a circuit arranged for the measurement 
of E.M.F. by the compensation method.* 
The silver rods were first of all kept at a red heat for several 
hours. In this condition the surface of the rods had a crystalline 
frosted appearance, and the rods were quite soft. The cell 
Agi Ag -^^ 3 — Ag n , where Ag : , Ag n represent the two rods of 
silver, had no E.M.F. One of the rods (Agi) was then polished 
with emery-papers of different degrees of fineness, and then wiped 
with a clean cloth. The surface of the polished rod, when 
examined under the microscope, appeared smooth and unbroken, 
save for a few scratches, which are hard to avoid in polishing such 
soft material. The cell Ag r — AgN0 3 — Ag n now showed an E.M.F., 
the polished rod, Ag x , being the negative pole. The polished 
metal has evidently a greater tendency to dissolve than the 
annealed rod. The average E.M.F. of the cell is 0‘013 volt. It 
is difficult to obtain always exactly the same value ; the smallest 
I have obtained is ’008 volt, and the largest '020 volt. 
The E.M.F. remains practically constant; if the cell be short- 
circuited for a day and then released, the E.M.F. rises in a few 
minutes from zero to within *002 volt of its former value. 
I have used sodium nitrate solution ( t §-q) and sulphuric acid 
(•05 per cent.) as electrolytes instead of silver nitrate ; in both cases 
the E.M.F. was in the same direction and of approximately the 
same magnitude as in the case of silver nitrate, but the values 
obtained varied a great deal. 
Returning to the case where silver nitrate is used as an 
electrolyte, the polished rod of silver (Ag x ) was treated with nitric 
acid for a few seconds ; it was then washed with water, and 
warmed to about 200° C. for a short time. The vitreous skin was 
thus dissolved away, and the E.M.F. of the cell Ag x — AgN0 3 — Ag n 
was zero. 
* Ostwald -Luther, Physiko-chemischc Messungen, 2nd ed., p. 367. 
