( 458 ) 
valve require no further explanation) to the tube Ho, in which 
the pressure is indicated by the manometer, and further along A, 
to Z. When the safety-valve C3, rises the canals Cy and Cy make 
a communication between the exhaust chamber behind C,, (fig. 4) 
and the forcing chamber behind C;. This safety-valve is pressed 
by a long spring C33 with adjusting block C,, and adjusting pin 
C,,. The adjusting pin passes through the packingbox C,,, ends 
in a square head ©,,, and is adjusted for a given pressure by means 
of a wrench. If this is done we can allow the auxiliary compressor 
to work whether the mercury pump takes up the compressed gas 
or not. 
Chemistry. — “The alleged identity of red and yellow mercuric 
oxide’. Part IL. By Dr. Ernst COHEN (Communicated by 
Prof. H. W. Bakuuis RoozeBoom). 
(Read January 27, 1990.) 
1. It has been stated in my first communication !) that there 
exists between red and yellow mercuric oxide a difference in free 
energy 0.685 millivolts at 25.°0. 
I now wish to communicate some details as to the determination 
of the temperature coefficient of the previously described mercuric 
oxide cell and discuss the thermic determinations made by VARET 
im 1896 2): 
2. The E.M.F. of the mercuric oxide cell may be represented 
by the equation : 
Ee dn 
eg ade 
a aa adv 
in which z is the E.M.F. of the cell at the temperature 7, Z, the 
chemical energy of the process taking place on the passage of «, 
Coulombs, whilst » represents the valency of the mercury. 
dn 
dE 
rj 
Tf “n° and” T “and” also the temperature coefficient 
known we can calculate Z, with the aid of the said equation. 
1) Proc. Royal Acad. Nov. 25, 1899, pg. 273. 
*) loc. cit. pg. 278, note 1. 
