( 278 ) 
If, in accordance with the completely harmonious determinations 
of KAHLE?) and CALLENDAR and BARNES?) we take the E.M, F. 
of the Crark-cell at 25°,0 as 1,4202 volts, then the E.M. F. of 
the Weston-cell at 25°,0 is, according to these determinations, 1,0184 
volts. The comparisons made by JAEGER and WaAcHsMUTH ®) also 
give the E.M.F. of the Wesron-cell at 25°,0 = 1.0184 volt. 
Our standard was thus very trustworthy during the whole research. 
An extremely sensitive THOMSON mirror galvanometer was used 
as zero instrument. The thermometers employed were compared with 
a thermometer standardised by the Physikalisch-Technische Reichs- 
anstalt. 
The thermometer which indicated the temperature of the ther- 
mostat was graduated in */1o. 
7. A series of measurements was first made with the red and 
yellow oxides from Merck, which were used without further treatment, 
after it had been found that no impurities could be discovered in 
them by analytical means. 
I first determined the potential difference red HgO — red HgO 
by setting up an element arranged thus: 
Hg — red HgO — potash lye — potash lye — red HgO — Hg. « 
The potential difference was less than 0,000001 volt 4). 
In the same way the potential difference of yellow HgO — yellow 
HgO was measured. This also was less than 0,000001 volt. 
8. The determination of the potential difference red HgO —- 
yellow HgO then followed. An interesting phenomenon, which might 
have been foreseen, was here encountered. 
It is known that the two oxides behave differently towards a 
number of reagents, the velocity with which they react being 
different. The yellow oxide, for example, dissolves in acids more 
rapidly than the red. 
If now, as might be anticipated, the equilibria of the two oxides 
with the solution in the cell are attained with different velocities, 
it is to be expected that the E.M.F. of the mercuric oxide cell, 
regarded as a function of the time, will increase to a certain maxi- 
1) See Thätigkeitsbericht der Physikalisch Technischen Reichsanstalt 1896/97. Zeit- 
schrift fiir Instrumentenkunde 17, 144 (1897); WrEDeMANN's Annalen 64, 94(1898). 
Zeitschrift fiir Instrumentenkunde 1898. 161. 
2) Proc. of the Royal Society 62, 132 (1897). 
8) Wrep. Annalen 59. 575 (1896) en Zeitschrift für Instrumentenkunde 1898, 161, 
4) The smallest quantity measurable. 
