GALVANISM 59T which are filled with water acidulated with sulphuric acid, the same fluid being contained in the shallow vessel. The electrodes are placed as near together as practicable, in order to reduce the resistance. The electrolysis of FIG. 24. Voltameter. pure water is difficult, and it is doubtful if it occurs in the presence of sulphuric acid, ex- cept at the surface of the positive electrode, it being more probable that the chain of mole- cules of sulphuric acid lying between the elec- trodes have their elements displaced and re- joined in the following manner : JA H. S0 4 H 2 S0 4 H 2 Other binary compounds are very readily de- composed by electrolysis, as for instance iodide of potassium, KI, in this manner, IK I K I K I K L the negative element of the compound mole- cule, iodine, going to the positive, and the positive element, potassium, going to the nega- tive electrode. By means of a battery of 250 couples Sir Humphry Davy decomposed the alkalies potash and soda, showing them to be oxides of two hitherto unknown metals, potas- sium and sodium. The electrolysis of potash may be accomplished by placing a globule of mercury in a cavity made in a piece of caustic potash, which being moistened lies upon a piece of platinum foil. The latter is connected with the positive electrode of a Grove's or Bunsen's battery of six or seven cells, and the point of a wire, connected with the negative electrode, is placed in the mercury globule, which then at- tracts the positive element, metallic potassium, of the caustic potash, while the negative ele- ment, oxygen, is attracted by the plate of plati- num. The potassium on being liberated forms with the mercury an amalgam, which on being subjected to distillation yields metallic potas- sium. Any substance which is separated into component parts by electrolysis is called an electrolyte, and must be in a fluid condition so as to admit of displacement of the molecules. Although it is believed at the present time that the molecules of solid bodies do not touch each other, still it must of necessity be a condition that they are held by their polarity in such fixed relations that no external force is suf- ficient to release them until the attraction is diminished by heat or by some solvent. From the nature of molecular combination, one con- stituent of a molecule is electro-positive, and the other electro-negative; therefore, when subjected to the influence of electrical attrac- tion, one of the constituents will be drawn to the positive and the other to the negative elec- trode of the battery. The positive electrode of a battery was named by Faraday the anode, and the negative the cathode, the former re- ferring to the upward and the latter to the downward direction of the current. The ele- ments or constituents of the electrolyte he called ions, the one going to the anode the anion, and the one going to the cathode the cation. Potassium, which is the most electro- positive of all bodies, is therefore always a cation, and oxygen, the most electro-negative body, is always an anion. As a general rule, the metals, or those constituents which contain them, are cations, while the non-metallic con- stituents are anions ; but some elements may be sometimes cations and sometimes anions. In chloride of sodium, NaOl, for instance, chlorine is an anion, while in chloric acid, OlaOs, it becomes a cation, or electro-positive element. In the electrolysis of ternary salts, which were formerly supposed to be consti- tuted by the union of an acid with an oxide of a metal, it was held that there was a separation of this oxide from the acid, and then a subsequent separation of the oxygen and the metal, and their appearance respec- tively at the positive and negative electrodes ; but this theory was only adapted to the one entertained with regard to the constitution of a salt. It is now held that ternary salts have in reality a binary constitution, the metal forming the electro-positive, and the salt radi- cal the electro-negative constituent. The elec- trolysis of a salt may be shown by using a U-shaped tube as represented in fig. 25. A solution of the salt colored with sirup of vio- lets is introduced into the tube, and the plati- FIG. 25. num electrodes of a battery are placed one in each leg. After the current has passed for a time the fluid will have a red color about the positive, and a green color about the negative electrode, the red being caused by the action
