296 
ON VOLTA-ELECTRIO INVERSION. 
down the phenomena reappeared in reverse order, the point of 
inversion, however, not being reached till a lower temperature 
than with an ascending temperature, owing, probably, to the 
concentration of the acid by evaporation. 
6. During the first half of the operations, or while the 
current was flowing through the liquid from iron to copper, the 
iron pole dissolved in the acid, with evolution of hydrogen bubbles 
at the copper pole. During the latter half of the operation, with a 
temperature above 150° C., the copper dissolved in the acid, and 
bubbles, believed to be chiefly of sulphurous acid gas, arose freely 
from the iron pole, which also became covered with a black- 
looking crust. At the temperature at w^hich inversion occurred, 
hubbies appeared to be rising in small quantities from both poles, 
but in such small quantities that it was impossible to ascertain 
their nature. 
It appears then that at the degree of concentration specified 
sulphuric acid when cold attacks iron more readily than copper ; 
but attacks copper more readily than iron when heated above 
150° ; that metal w^hich at any given temperature is the more 
readily attacked serving as the electropositive metal or negative 
pole of the pair. 
7. The currents in question appear to be true voltaic currents, 
because (i.) their production is accompanied by chemical action 
in the cell ; (ii.) because the electromotive forces producing them 
are far greater than the thermo-electromotive forces which would 
be produced by the contact of the metals at those temperatures, 
though, of course, such thermo-electromotive forces were present 
and contributed to the somewhat complicated phenomena. The 
thermo-electromotive force of an iron-copper couple is only 
14 millionths of a volt for a difference of temperature of 1° C. 
between the junctions. 
8. A further consequence of this discovery is that a battery 
can be made of copper, iron, and sulphuric acid without there 
