ng
place; but the hydrogen, which in this case would have been liberated at
the copper plate, at once united with oxygen there, which it got by
decomposing the copper sulphate: hence water was formed, and copper was
deposited upon the copper plate; and, being an excellent conductor, the
battery would keep up a strong action for a long time.

Mr. Grove, also of London, in 1839 invented a battery which still goes
by his name, in which the hydrogen plate is of platinum immersed in
strong nitric acid, enclosed also in a porous earthen cell; and this, in
turn, is plunged into a vessel containing dilute sulphuric acid and the
zinc. In this case the liberated hydrogen immediately decomposes the
nitric acid, which readily parts with its oxygen; water is the product,
as in the other case, and the nitric acid loses strength. Strips of
carbon have been substituted for the platinum, and this is called the
Bunsen battery. It is otherwise like the Grove battery; it gives a very
powerful and constant current and it is by the use of one or the other
of these batteries, that most of the experiments in electricity are
performed in institutions of learning, and, until lately, most in use
for telegraphic purposes.

OTHER MEANS FOR GENERATING ELECTRICITY.

THERMO-ELECTRICITY.

IF two strips of different metals, such as silver and iron, be soldered
together at one end, and the other ends be connected with a
galvanometer, on heating the soldered junction of the metals it will be
found that a current of electricity traverses the circuit from the iron
to the silver. If other metals be used, having the same size, and the
same degree of heat be applied, the current of electricity thus
generated will give a greater or a less deflection, which will be
constant for the metals employed. The two metals generally employed are
bismuth and antimony, in bars about an inch long and an eighth of an
inch square. These are soldered together in series so as to present for
faces the ends of the bars, and these often number as many as fifty
pairs. Such a series is called a thermo-pile. This method of generating
electricity was discovered by Seebeck of Berlin in 1821, but the
thermo-pile so much in use now in heat investigations was invented by
Nobili in 1835. The strength of this current is not very great, a single
Daniell cell being equal to nine pairs of the strongest combination yet
discovered, namely, the artificial sulphuret of copper with Germa