Page:The American Cyclopædia (1879) Volume VII.djvu/603

 GALVANISM 591 fresh quantities of acid. Two ways have been devised to remedy this difficulty, one of which is using a siphon connected with the bottom of the cell, by which the strong solution of sulphate of zinc which settles may be dis- charged when fresh quantities of sulphuric acid Fia. 8. Cell of Grove's Battery. are poured in. The other method, devised by Buff, consists in an arrangement by which the zinc plate is gradually made to sink in the fluid at a rate sufficient to counterbalance the loss sustained by diminished chemical action. Grove's battery is a modification of Daniell's, in which nitric acid takes the place of sulphate of copper, and a plate of platinum that of cop- per. Fig. 8 represents a cell of Grove's bat- tery. A glass or earthen vessel, A, containing dilute sulphuric acid, receives a cylinder of zinc, within which is a porous earthenware cup, V, containing strong nitric acid, and in which there is immersed a platinum plate, P. A cover attached to it confines the fumes of hyponi- tric acid, N 2 4, which are liberated by the de- composing nitric acid, N" 2 O 5 . The chemical ac- tion is shown in the following diagram : Platinum Porous partition. tinum I I I Zinc plate. |N a O 4 O N 2 O 4 O I H 2 SO 4 H 9 SO 4 1 plate. -|+ -+-I+- + -I + Bunsen's battery is a modification of Grove's, which was devised by Bunsen, then professor of chemistry at Marburg, in 1843. The platinum element is replaced by carbon, which is an ex- cellent conductor, and more electro-negative than platinum. The graphite carbon taken from the retorts of gas works is often used ; or a cake may be formed by calcining in an iron mould a mixture of coke and bituminous coal. The electrolytic fluids may be the same as in Grove's ; but others are often used, as bichro- mate of potash, sulphuric acid, and common A battery may be charged with these fluids in the following manner : Two ounces of bichromate of potash are dissolved in 20 ounces of hot water, and when cold 10 ounces of sul- phuric acid are added. When the heat caused by the addition of the acid has subsided, the solution is placed within the porous cup, and a 345 VOL. vn. 38 saturated solution of common salt poured into the outer compartment, which is occupied by the zinc plate. The chemical action when these solutions are used is rather more com- plex than that which has been given for other fluids. The action of the sulphuric acid on the bichromate of potash produces sulphate of pot- ash and chromic acid. Hydrogen is evolved, and acting upon the chromic acid reduces it to chromic oxide, water being at the same time formed, while the chromic oxide combines with sulphuric acid, forming sulphate of chro- mium. The electromotive force resulting from these reactions is very great, and the batteries in which these fluids are used are now generally preferred where compactness is desired. La- clanche's battery consists of a carbon electrode packed in a mixture of peroxide of manganese and carbon or coke in coarse powder in a po- rous cell, and outside of this a cup containing a zinc electrode immersed in a solution of sal ammoniac. Its electromotive force is about nine tenths of that of Daniell's. The compara- tive values of the electromotive forces of seve- ral forms of batteries are as follows : Bunsen's, 839 ; Grove's, 829 ; Daniell's, 470 ; Smee's, 210 ; Wollaston's, 208. Electricity developed iy the Action of Solutions upon one another. If two vessels, one containing a solution of potash and the other of nitric acid, are connected by a bundle of asbestus, as represented in fig. 9, and two platinum plates are connected with the wires of a galvanometer, a gal- vanic current will pass through the lat- ter instrument from the acid to the alkali. Davy supposed that this effect was due to the difference in action of the two liquids upon the platinum; but if two cups, each containing a solution of the same salt, as nitre, are placed one on either side of the first named cup, and connected with them by bundles of asbestus, and the platinum slips connected with the galvanometer are placed in the solution of nitre, a current will be pro- duced flowing in the same direction as in the first instance, but it will be weaker, because of the increased resistance offered by the addi- tional liquid. The action of one acid upon another will also generate galvanic currents. Let strong nitric acid be placed in one branch of a U tube, and strong sulphuric acid be care- fully introduced into the other so as not to mix (for which purpose a dividing membrane may be used), and platinum strips connected with the wires of a galvanometer be placed one in either branch ; a current will thus be gene- rated, passing from the sulphuric acid through the galvanometer to the nitric acid. Fig. 10 represents a galvanic couple composed of two liquids and one metal, devised by Becquerel, and called an oxygen circuit. A bottle, d, contains nitric acid, and into its mouth is in- serted a tube containing a solution of caustic FIG. 9.