160 PHYSICS. 
extremity of the carbon cylinder is turned off conically, and upon it is firmly 
fixed a zinc ring, a, which carries a hollow zinc cylinder, c, by means of the 
bow, 6. This zinc cylinder dips into the clay cylinder of the followmg 
element. fig. 63 exhibits in plan the connexions of the elements of a carbon 
battery, being a combination of four pairs. Here p will be the positive pole, 
and n the negative. 
Among the different galvanic arrangements just described, we may 
distinguish three modifications, whose theory we shall now proceed to 
explain. The first consists of the pair of metallic plates immersed in a 
single liquid, and connected externally to the fluid by a metallic conductor, 
as a copper wire. Let us suppose the fluid to be dilute sulphuric acid. 
Here the water of the acid is decomposed at the same time with the 
electricities of the metals, and a current of positive electricity passes from 
the zinc through the fluid to the copper, thence through the connecting 
conductor back to the zinc. Negative electricity also passes from the zinc 
through the connecting conductor to the copper, and back again through 
the fluid to the zinc. If the connecting wire be severed, the positive 
current will make its appearance at the portion attached to the copper 
plate, and the negative at that to the zinc. The extremities of these wires 
thus attached to the plates are called the electrodes or poles of the battery. 
The oxygen of the decomposed water unites with the zinc, forming an oxyde, 
and this, with the sulphuric acid, forms sulphate of zinc. The hydrogen 
is carried to the copper plate and there liberated. 
The second modification is that in which two fluids are separated by a 
porous partition, one of thern a solution of a metallic salt, as sulphate of 
copper. This is the case in the constant batteries of Daniel and Becquerel. 
Here the water of dilute sulphuric acid is decomposed by the current, 
oxygen being liberated at the zinc plate, and uniting with the zinc, forming 
an oxyde ; the sulphuric acid then converts this into sulphate of zinc. The 
hydrogen carried with the positive current through the porous partition 
assists in decomposing the sulphate of copper, combining with the oxygen 
of the copper, and liberating sulphuric acid and metallic copper. | 
The third modification is seen in Grove’s and Bunsen’s batteries, where 
nitric acid replaces sulphate of copper ; dilute sulphuric acid here, as in the 
Jast modification, forms the second fluid. The action of tne oxygen of the 
decomposed water is the same as in the last case. The hydrogen passing 
through the porous partition unites with the nitric acid, takes from it an 
atom of oxygen and forms water, leaving nitrous acid, as shown by the 
deep red fumes produced. 
The dry pile still remains to be mentioned, a Voltaic arrangement, in 
which every two pairs of metallic substances are separated, not by a fluid, but 
by adry solid body. Of these the dry pile of Zamboni is best known, and 
consists of a great number of disks of gold and silver paper, superimposed in 
pairs, with their metallic faces in contact, and with the same metal always 
uppermost. Here the paper, being always slightly damp, supplies the place 
of a fluid conductor. The paper may also be covered with other metallic 
substances than gold and silver, and is best cut out by a punch. The 
334 
