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 German silver. MAGNETO-ELECTRICITY. [Illustration: FIG. 2.]