ELECTRO-CHEMISTRY. 776 ELECTRO-CHEMISTRY. tron, forming the compound H Q), wliile an atom of oxygen in that state forms the com- pound (just as water is llOU). When not thus combined with the atoms of other ele- ments, electrons may possibly form neutral mole- cules © 0. The substance made uji of ?uch molecules may have appreciably no weight, may be a non-conductor, and may be capable of elec- tric polarization; and it is worthy of notice that these are precisely the properties attributed to the luminiferous ether. While this hypothesis lenders Faraday's law (sec Ki-ECTUIcity ) a spe- cial case of Dalton'.s law of multiple proportions, and may certainly be helpful in fixing on the mind a scmiewhat definite image of the mecha- nism of electrical processes, it can hardly lie said yet to rest on a sutlicicnt foundation. Whether It should be accepted or not, will have to be de- cided on the principle expressed, some years ago, by I'oincarc: "It is nonsense to ask of a theory, 'Is it true or false?' The question can only be, 'Is it fruitful or not!'" At any rate, the electron hypothesis must not be allowed to confuse our conception of elec- trical energy and its relations to other forms of energ'. The term energy, when properly used, designates the cause of any change that is actu- ally taking place, or that is capable of taking place, in a given system. Thus we may speak of the energj' of a falling body, and even of a body suspended at a certain height from the earth's surface; for such a body, if released, uould fall to the earth. Similarly, we may speak of 'elec- trical energj' whenever a transport of electricity from one point to another is possible in a given system. Energy of every known form can be re- solved into two factors — a 'capacity factor' and an 'intensity factor.' In the case of electrical energy, the capacity factor is generally known as 'quantity of electricity,' the intensity factor as 'potential-difference.' Just as in the case of a suspended body the amount of gravitation energy is the greater, the greater the mass of the body and the greater the difference of level between the body and the earth's surface, so in the case of Si possible (low of electricity, the amount of electrical energy is the greater, the greater the quantity of electricity and the greater the dillVr- ence of potential between the two given points. Whatever be the ultimate nature of a mass of matter, gravitation energy is still a form of energy. Similarly, whatever be the ultimate nature of a quantity of electricity — whether elec- tricity be a kind of matter or not — electrical energj' is still a form of energy, capable of Iwing transformed into kinetic energj-, chemical energy, etc. Ki.ECTRO-CitEMiCAi. Eeaotion'S. A system in which chemical energy is transformed into elec- trical is termed a galvanic or voltaic cell, and a combination of two or more cells is termed a galvanic or voltaic battery. One of the first questions that naturally suggest themselves to the student of electro-chemistry is. What are the characteristics of those chemical changes whose energy may assume the electrical form? In other words. What chemical reactions can be utilized for the construction of voltaic cells? The an- swer is, All changes, and only such changes, in which acids, hases, or salts, take part, or may he caused to take part, can he employed electro- chemically. Substances of these classes are termed 'electrolytes,' and they are usually cm- ployed, in cells, in the form of aqueous solutions, in which their molecules are assumed to be disso- ciated into fragments, termed 'ions' (see Disso- CI.T10N). that are charged with cither positive or negative electricity. In most cells the chemi- cal action consists, on the one hand, in the ^olution of one of the metallic electrodes, its particles entering the solution in the forms of ions; and on the other hand, in the deposition upon the other electrode of another metal, whose particles formed the ions of the solution. For ex- ample, the Daniell cell consists, on the one hand, of a zinc electrode immersed in a solution of zinc sulphate, and on the other hand, of a copper electrode inmiersed in a solution of copper sul- phate, the two solutions being in contact, and the two metals being connected by a metal wire; the chemical action of the cell consists, on the one hand, in the entrance of zinc into the solu- tions in the form of zinc ions, and on the other band, in the deposition, on the copper electrode, of copper ions in the state of metallic copjier. The two simul',^aneous changes evidently ensue in the system at two separate points connected with one another by a suitable conductor of elec- tricity. This 'chemical action at a distance' is an essential characteristic of elect rochemical action. If in the Daniell cell the two chemical changes took place at points in immediate contact with each other (e.g. if zinc were caused to replace copper by being simply placed in a solution of copper sulphate), a certain amount of heat, but no electricity, would be produced. An excellent example of changes in which elec- trolytes may and may not take part is presented by the mere How of hydrogen or oxygen gas, at constant temperature, from places of higher to places of lower pressure. If, say, hydrogen gas is contained in two separate vessels under un- equal pressures, a flow of gas will naturally take place from one vessel into the other, if com- munication is established between them. But the equalization of pressures can also l)e caused to take place as follows: let two bars of platinum coated with 'platinum black' (finely divided platinum, which has a great capacity for occlud- ing gasps) be half immersed in dilute solution of sulphuric acid, while the other halves are ex- posed to hydrogen gas contained in two vessels, under two different pressures : on connecting the platinum bars outside the solution by a metal wire, it will be found that a current of electricity passes through the latter, while hydrogen passes through the solution until the pressures in the two vessels have been equalized. The action thus taking place is as follows: in dilute aqueous solution, the molecules of sulphuric acid are broken up into electro-positive hydrogen ions H and electro-negative ions SO,: neutral hydro- gen molecules from the vessel of greater pres- sure enter the solution in the form of hydrogen ions. and. replacing the free hydrogen inn* of the acid, drive them from the solution in the form of neutral molecules into the vessel of lower pressure. Evidently here, too. as in the case of any other voltaic cell, the electric current is caused by ions taking part in the change. Eleotro-Motimc Force. We can now discuss the question as to what determines the electro- motive force of a voltaic cell. i.e. the difference of potential existin<r between its two electrndes. It was once believed that a considerable differ-
