530 GROVE'S GAS BATTERY. [264. It has been already stated that the amount of force set in motion in a voltaic arrangement depends upon the difference between the attraction of the two metals for the active principle or radicle of the acid. Under circumstances favourable to the production of a current, decomposition of the liquid which excites the action always occurs; the elements of the liquid are separated from each other, and they either combine with the metallic plate, or else they accumulate upon its surface, giving rise to the condition of the plates which is often described under the inappropriate term polarization of the plates or electrodes. These adhering substances oppose the voltaic action and enfeeble it, owing to the tendency of the separated components of the liquid to re-unite. When, for example, diluted sulphuric acid is used, it becomes a desideratum to get rid of the hydrogen which adheres to the platinum, and produces a current in the opposite direction. The existence of this counter-current may be rendered evident by connecting with one end of the wire of a galvanometer a platinum plate which has thus been opposed to a plate of zinc: on attaching to the other end of the galvanometer wire a second but clean platinum plate, and plunging both into diluted acid, a powerful deflection of the needle will be observed. This observation has been ingeniously applied by Grove, who has constructed what he terms a gas battery, by opposing a plate covered with oxygen to the FIG. 217. plate coated with hydrogen, whilst at the same time he increases the surfaces of contact between the platinum and the oxygen and hydrogen. Fig. 217 represents a cell of this battery. It consists of two tubes, o and H; through the upper extremity of each passes a platinum wire, which is fused into the glass, and attached to a platinum plate sufficiently long to reach to the bottom of the tube. The surfaces of these plates are coated by means of voltaic action with finelydivided platinum, for the purpose of increasing the surfaces of contact between the metal and the gas. The tube H has double the capacity of the tube o. These tubes are supported in the vessel, s, by the plug through which they pass. In order to use the apparatus, the vessel s is filled with diluted sulphuric acid, and by inverting the cell the tubes are likewise filled with the liquid. The plates in the tubes o and H are then connected by the mercury cups at top with the wires of a voltaic battery in action, so that by the decomposition of the diluted acid the tube o shall become filled with oxygen and the tube H with hydrogen. The tubes having been thus filled, the battery wires are withdrawn. If the mercury cups at the top of the tubes o and H be now connected with the wires of a galvanometer, powerful deflection of the needle will be produced, and a current will be maintained through the apparatus in the direction of the arrows. The two gases will gradually diminish in volume, and will in a few days 264.] GROVE'S GAS BATTERY. 531 entirely disappear, but the current will be maintained so long as any portions of the gas remain uucombined. By connecting eight or ten such cells in succession, so that the oxygen tube of one cell shall be connected with the hydrogen tube of the adjacent cell, sparks may be obtained between charcoal points, and various chemical decompositions may be effected. The polar chain by which these changes are produced may thus be represented by symbols: H,SO,, indicating a molecule of diluted acid, O and H, representing the disturbing atoms of oxygen and hydrogen: OH, SO, H, SO, H, SO H ̧ The brackets above the row of symbols are intended to show the molecular arrangement before the circuit is completed; those beneath the symbols show the action during the passage of the current. Since no action occurs in the gas battery until metallic communication between the plates is effected, it appears that the use of the platinum plates consists in favouring the action by condensing the gases upon their porous surfaces, and in acting as conductors of the current. It may indeed be stated generally, that the accumulation of either of the elements of the exciting liquid upon the metallic plates of a voltaic combination, always tends to produce a countercurrent, and therefore reduces the efficiency of the combination to a proportionate extent. Hydrogen is the element which, in the usual mode of experiment, principally accumulates upon the negative plate, so that any contrivance by which the adhering hydrogen is removed, exalts the energy of the circulating force. This removal of the hydrogen may be effected by means which act either on chemical or on mechanical principles. The chemical principle is the most perfect. It consists in adding to the liquid a compound which has a tendency to unite with the hydrogen; hence the energy of the current is much increased by mixing a little nitric acid (HNO,) with the exciting liquid, comparatively little hydrogen being set free in this case * The same end is attained by adding to the sulphuric acid a solution of some of the metallic salts, such, for instance, as cupric sulphate (CuSO4). When cupric sulphate is employed, metallic copper is deposited upon the negative or conducting plate, whilst the sulphion with which it was previously united combines with the zinc. A disadvantage, however, is experienced when the liquid which absorbs the hydrogen is in contact with the zinc, and this is particularly evidenced when cupric sulphate is used. The zinc acts at once = *By the action of hydrogen on nitric acid, peroxide of nitrogen (N,O,) and water are formed, thus: 2HNO, + H, N ̧O̟ ̧ + 2H ̧O; and both these substances are dissolved by the nitric acid. 532 LOCAL ACTION-OBVIATION OF COUNTER-CURRENTS. [264. on the solution of copper, and becomes coated with reduced copper hence, between the particles of zinc and those of the reduced copper innumerable small circuits are produced, which occasion a violent discharge of hydrogen from the entire surface of the generating metal, or rather from the copper deposited upon it; but the zinc thus dissolved contributes nothing to the general effect; it becomes merely a case of local action (260). This experiment with the cupric sulphate throws light upon the cause of the effervescence which takes place when common zinc is treated with diluted sulphuric acid. Commercial zine always contains lead and other foreign metals mixed with it in very appreciable quantity; these act as dischargers to the hydrogen, and give rise to numerous local circuits at all points of the surface of the zinc. Perfectly pure zinc is dissolved very slowly in acid for want of these discharging points, but the acid is not absolutely without action upon the metal. Any inequality in susceptibility to chemical action gives rise to a current between two substances suitably disposed; hence any difference in density between two pieces of the same metal may suffice to cause a current; and a piece of hammered zinc will generally act as a conducting plate to a piece of zinc well annealed. The adherence of a film of oxide or of fatty matter to the surface of one piece will also cause a difference, and hence two pieces of metal which may even have been cut from the same strip may, under certain circumstances, produce a feeble current. The inconvenience which is occasioned by local action, when nitric acid or cupric sulphate is mixed with the liquid which is in contact with the zinc, may be avoided by the employment of porous diaphragms; and if the zinc or generating plate be plunged into diluted sulphuric acid, whilst the platinum or conducting plate is made to dip into the nitric acid or into the solution of cupric sulphate, which is separated from the generating plate by means of a tube of porous earthenware, combinations of great efficiency are obtained. (265) Daniell's Battery.-These important facts were first clearly enunciated by Daniell. Their application to the voltaic battery enabled him to detect the cause of the rapid decline in the activity observed in all the forms of batteries which up to that period had been devised, and they led him to the invention of an arrangement which not only obviated these defects, and enabled him to keep up a current of constant strength for many hours, but also furnished electrical science with a battery of far greater activity for its size than any which had previously been used. 265.] DANIELL'S CONSTANT BATTERY. 533 Fig. 218 exhibits a section of one of the cells of Daniell's combination. The outer case, c, consists of a cell, or cylinder of copper, which is constructed so as to retain liquids, and is filled with a solution of cupric sulphate, B, acidulated with an eighth of its volume of sulphuric acid. The solution is kept saturated with the salt by means of crystals of cupric sulphate, D, which rest upon the perforated shelf, F. In the axis of the cell is placed a tube of porous earthenware, E, filled with an acid solution, a, which consists of I part of oil of vitriol diluted with 7 parts of water. A rod of amalgamated zinc, z, is placed in this tube. On making a metallic communication between the zinc rod and the copper cell, a voltaic current is established: and by employing twenty or thirty cells of this description, always connecting the zinc of one cell with the copper of the next, a combination of great power is obtained. FIG. 218. The following diagram may serve to explain the manner in which the force is transmitted through the cells :-The diluted sulphuric acid may be regarded as a compound of hydrogen with sulphur and oxygen, and is represented as H2SO,; whilst cupric sulphate may be looked upon as a compound of copper with the same compound of sulphur and oxygen, and is indicated by the formula CuSO,. Let the brackets above the row of symbols represent the connexion of the particles which compose the liquid before contact is made between the plates Cu and Zn at the ends. The alteration in the molecular arrangement of the liquid which occurs after the connexion is made between the copper and the zinc, may be represented by the altered position of the brackets beneath the line A, which divides the symbols of the cupric sulphate from those of the sulphuric acid, in this case represents the porous diaphragm : A Cu Cu SO, Cu SO. | H, SO, H, SO, Zn The result of the action is, that so long as the contact between. the metallic plates is maintained, zincic sulphate is formed uninterruptedly in the porous tube, whilst a continual deposit of a corresponding quantity of metallic copper takes place upon the internal surface of the copper cylinder. Fig. 219 shows a convenient and inexpensive form of Daniell's battery. The solution of cupric sulphate is contained in glass or earthenware jars 7 inches (18 centimetres) high and 3 inches (9 centimetres) in diameter. The copper plates consist merely of rectangular sheets of copper, one of which is represented at ; they are bent in a cylindrical shape and placed in the jars. By means of the strip b, each plate is easily connected with the zinc rod of the adjacent cell, and made fast to it by the binding-screw c. The colander, for the support of the 534 DANIELL'S BATTERY. [265. crystals of cupric sulphate, rests upon three or four little pieces of copper, which are made to project inwards upon the sheet, at a suitable height, as shown at d. At B, several cells of the battery are represented as arranged in a consecutive series. Twenty such cells compose a battery adequate to the performance of almost any experiments on the chemical decomposition of bodies in solution. It is essential in mounting a voltaic arrangement of any kind, that the surfaces of contact between the metals be perfectly clean a film of oxide will materially impede the transmission of the current, and if the force in circulation be feeble, it may even arrest it altogether. As a precaution, it is better before connecting the different parts of the apparatus, to pass a file or a piece of emery-paper over all the surfaces of the copper, the zinc, or the other oxidizable metals which are to be placed in contact with each other. Surfaces of platinum, if well washed and dried, do not need friction with emery-paper. FIG. 220. 2 (266) Grove's Nitric-Acid Battery.-The nitric-acid battery, contrived by Grove, is a still more powerful combination on the same principle as Daniell's. It consists of a slip of platinum, P, fig. 220, which is plunged into the porous cell, N, and this is filled up with undiluted nitric acid. The outer cell, s, is filled with diluted sulphuric acid, and in this acid is placed a flat sheet of amalgamated zinc, z, bent so as to infold the porous cell. The acid liquid in s may be conveniently made of 1 measure of oil of vitriol diluted with 4 measures of water. This combination presents in a small compass the principal desiderata for attaining |