Electromotive Forces in the Voltaic Cell. 279 
Additional Note, March 13th, 1885. 
On page 176 of the March Number of the Philosophical 
Magazine I quote from Von Zahn and others an extraordinary 
statement, ascribed to Professor Exner, that a thermopile ceases 
to work ina vacuum; saying at the same time that I had been 
unable to find the statement myself in Hxner’s writings. For 
a sufficient reason as it now turns out, because it is not there. 
Turning over some back volumes of ‘ Nature’ recently, I 
accidentally came across a letter from Prof. Exner in vol. xxii. 
p- 170, December 1880, denying that he had ever said sucha 
thing, and maintaining that his memoirs were quite clear on 
the subject of the relation, or rather want of relation, between 
voltaic and thermoelectric phenomena. This is perfectly true ; 
Prof. Hxner has expressed himself on this subject with decision, 
and finds fault with Prof. Edlund for ever confusing the two 
distinct. things together. 
In the same volume of ‘ Nature,’ on page 312, appears a 
note from Professor Young acknowledging that he ought to 
have been more careful to refer to the original statement, and 
giving one of the “ Physical Notes” of ‘ Nature,’ p. 156, 
vol. xxil., as his authority. This “ Physical Note’’ quotes 
from /’ Hlectricité ; and so the error, however originally manu- 
factured, gets handed on from one paper to another. 
One of the memoirs which I have unfortunately overlooked 
in my historical sketch of the experimental work done in 
connection with the subject has come to hand this morning, 
through the kindness of the authors, MM. H. Bichat and R. 
Blondlot, of Nancy. It was published in the Comptes Rendus 
(1883), t. xe. pp. 1202 et 1293, and in the Journal de Physique, 
2° série, li. p. 533, and is an attempt to measure the Volta- 
effect for two liquids. The method employed involves the use 
of three vessels, A, B, C, of which A and C contain one liquid 
and B the other. A and B are then connected by a liquid 
siphon with porous diaphragm | the capillary H.M.F. possibly 
excited by this diaphragm is bad]; and the potential of the air 
between B and C is then rendered uniform |or, commonly 
speaking, the potentials of B and C are equalized | by making 
one of the liquids drip inside a cavity kept moist with the 
other. A and C are connected by platinum electrodes through 
a compensation resistance-box and battery to a quadrant- 
electrometer, and the difference of potential between them 
measured. The arrangement is highly ingenious, but the 
difficulties of definite and certain measurement must be 
considerable. 
