598 GALVANISM of the acid, and the green by that of the alkali, these bodies being formed subsequently to the electrolytic decomposition of the salt by the union of the metal with the oxygen of the wa- ter, and the union of the body SC>4 with the hy- drogen. It was the appearance.of these acids and bases at the electrodes wljich led to the old theory of saline compounds. In electroly- sis there is not merely a separation and recom- bination of constituents in the chain of mole- cules between the electrodes, but there is an actual passage of one to the positive and of the other to the negative electrode. This ac- tion was demonstrated by Davy in the follow- ing manner: A solution of sulphate of soda was placed in two vessels and connected by a bundle of asbestus moistened with the same solution. The electrodes of a battery being placed in the cups, it was after a time found that the cup connected with the positive elec- trode contained all the sulphuric acid, while the soda was all found in the other. If one of the cups contains a solution of sulphate of soda, and the other a solution of chloride of so- dium or of potassium, both metallic bases will after a time be found in the cup connected with the negative electrode, and the acid radicals in the other ; and this transference of molecules will take place with any number of solutions, and through any number of cups which will not offer too great 'resistance to the passage of the current, provided that all the compounds which are formed by the inter- change of molecules are soluble. If, however, an insoluble compound is formed by the union of any of the constituents in any of the ves- sels, it will be precipitated and thus finally eliminated from the solution. Thus if a solution of sulphate of potash is placed in A, fig. 26, con- nected with the neg- ative electrode of a battery, distilled water in 0, connected with the positive electrode, and a solution of caustic baryta in B, all being connected by threads of moistened asbestus, the passage of a current will effect no transference of sulphuric acid to the positive electrode, but a precipitate of sul- phate of baryta will be formed in the cup B. The researches of Faraday have established the following laws of electrolysis: 1. Electro- lysis cannot take place unless the electrolyte is a conductor. "Water cannot be decomposed when in a state of ice, and other substances, as oxide of lead and chloride of silver, require for electrolysis to be fused to give them con- ducting power. 2. The energy of electro- lytic action is the same in all parts of the cur- rent. 3. The same quantity of electricity de- composes chemically equivalent quantities of all the electrolytic constituents through which it passes. That is to say, the same current will in the same time decompose 165 parts of iodide of potassium, 101 of nitrate of potash, 59 of FIG. 26. chloride of sodium, and 9 of - water. The de- composition which takes place in electrolysis creates a resistance to the current, and tends to generate a current in an opposite direction, the action being similar to what w,ould be the case if one of the cells of the battery were to be reversed, so that the current would be forced to pass through the liquid from the copper to the zinc. This is in agreement with the doctrine of conservation of forces. A certain amount of the power generated by the consumption of the positive element of the battery, that is to say, by the combina- tion of the acid with the zinc, may be expend- ed in one way or another in the conductors which are traversed by the current. A cer- tain amount of decomposition or electrolysis may be effected in one part, a certain amount of heat in another, and a certain quantity of mechanical power in another; the sum of all the forces expended being precisely equal to the original electromotive force. Polarization of Electrodes. After protracted electrolysis, if the electrodes are disconnected from the battery and placed in a conducting liquid and connected externally by a wire, a current will flow in a direction opposite to that which was generated by the battery. Suppose, for instance, FIG. 27. FIG. 28. that in fig. 27 a battery decomposes sulphate of soda by a current passing in the liquid from A to B. If after a time the battery is removed and the wires attached to the electrodes are connected and coiled around a magnetic needle as shown in fig. 28, it will be found that the current is now flowing in the liquid from B to A, or in a direction opposite to that urged by the battery. The action may be explained as follows : During electrolysis potash collects on the electrode B, and sulphuric acid on A. The battery being removed and a connecting wire substituted, the acid and alkali tend to unite and produce an electromotive force in an opposite direction. In the electrolysis of water, or any body which causes oxygen to be evolved at one electrode and hydrogen at the other, a thin film of gas becomes attached to either plate, having sufficient electromotive force to send a current in the contrary direc- tion when the battery is removed and a con- necting wire introduced. Such currents, pro- duced by polarized plates, are called secondary currents; and upon this principle Prof. Grove constructed a gas battery which is capable of producing a continuous current. Two glass tubes (fig. 29), closed at the top and each con- taining a strip of platinum of the length of the tube having a surface of finely divided plati-
