KLECTKOLYTES.] E L E C T 11 I C I T Y 47 f(>und that the solution continues to decompose so long as the cur rent passes, zinc appearing at the cathode, and chlorine at the anode. The metallic zinc precipitates, and the chlorine combines with the platinum of the anode to form platiuic chloride. It is obviously essential in an electrolyte that it should be a compound in some sense or other. It is not, however, true that all compound bodies are electrolytes. Fluidity is also a necessary condition, whether attained by heating to the melting-point, or by dissolving in water or other solvent. Faraday established as a law, to which there ap pear to be few, if indeed any, exceptions, tliat all sub stances winch in t/te solid state are very bad conductors, but conduct on being heated to the melting-point, are electrolytes, i.e., are decomposed by the passage of the electric current. Faraday thought that periodide of mercury, fluoride of lead, and some other bodies were exceptions to this law ; but later researches seem to have established that this is not -so. (Of. Experimental Researches, 414, 439, 1340, fec., and Wiedemann s Galvanismus, i. 191, <fcc.) The conductivity of electrolytes in solution also increases rather quickly with increase of temperature, while the conduc tivity of metallic conductors, on the other hand, diminishes, but more slowly, as the temperature rises. In considering the passage of the current through elec trolytes, it is convenient to distinguish two cases. First, let there be a steady, or at least permanent current, and a continuous evolution of the products of electrolytic decom position (these are called the "ions," anion and cation at the anodo and cathode respectively). The amount of ion that appears at an electrode in a second is equal to the strength of the current (supposed constant during a second) multiplied by a constant called the electrochemical equivalent of tlie ion The electrochemical equivalent is proportional to the chemical equivalent, account being taken of the " valency " of the ion. (See art. ELECTROLYSIS.) _ For instance, if C be the strength of the current in the illustra tive case above, then the amount of zinc deposited at the cathode in time t will be zCt, and the avnount of chlorine liberated at the anode cCY, where z and c the electrochemical equivalents of zinc and chlorine, and z : c :: ~- : 35 5, zinc being divalent. If a cell con taining ..ead chloride (PbCl 2 ) were also inserted into the circuit, the same amount of chlorine would be liberated at the anode, and the amount of lead precipitated at the cathode would be pCt, where 207 65 p : z : c :: -5- : -^ : 35 5, i.e. :: 103 5 : 32 5: 35 5 -i As the electrochemical decomposition ("electrolysis") goes on, the surface of the electrodes is altered. In some cases the ion is merely deposited on the electrode, in other cases it combines more or less intimately therewith; but in general there is an alteration of the nature of the con tact, and a consequent alteration of the electromotive force at the surface of the electrode. Experiment shows that this electromotive force, in a great many cases, tends to oppose the passage of the current. So that if we insert an electrolyte into any circuit, the current starts with a certain value, and falls more or less quickly, until it reaches a limit at which it remains steady. The opposing electromotive force of " polarization," as it is called, has then reached its maximum, and the deposition of the ions goes on without further alteration of the contact surfaces. It is obvious that this limit may be reached under a variety of different cir cumstances (vide infra, p. 86). There is also another pheno menon, the possibility of which we must not overlook, viz., an alteration of resistance, owing to the presence of the ions at the electrodes. This resistance, due to the ions, has been called the "transition resistance." The enfeebling of the current by the electromotive force of polarization might, as far as the observed result is concerned, be due entirely to an increase of resistance, or to n transition resist ance, and such was the explanation given by the earlier physicists. It is easy, however, to show that there is an . actual electromotive force of polarization ; for, if we dis engage our electrolytic cell from the battery, and connect its electrodes with a galvanometer, a current is indicated, which passes through the cell in the opposite direction to the original current. This could not be due to any tran sition resistance, but must arise from an opposing electro motive force generated by the passage of the battery current. This point can be illustrated by a hydrodynami- cal analogy. If we attempt to force water through a narrow capillary tube, or through a wide vertical tube against gravity, there is an opposing force in both case-;. But, when we remove the pressure, the water has a ten dency to return in the latter case, but none in the former. The former case represents a transition resistance, the latter an electromotive force of polarization. 1 Without denying the existence of a transition resistance, we see that an electromotive force of polarization actually exists. In some cases, e.g., amalgamated zinc in zinc sulphate, it is very small; in other cases, e.g., platinum electrodes in dilute sulphuric acid, it may considerably exceed the electromotive force of a DanielPs element. We have, up to this point, been treating the case where a permanent current finally flows through the electrolyte ; but there are cases where the existence of such a current would violate the principle of the conservation of energy. Suppose that a single Daniell s cell is the electromotor, then (see below, p. 90) if a current C is sent for a time t, an amount of energy dCt is absorbed in the cell, d being constant. Suppose, farther, that the excess of the intrinsic energy of the ions, in the state in which they are being delivered in the electrolytic cell, over that which they possess when in combination is w, then if a current (.! pass for a time t, an amount of energy u Ct will be evolved. But if vj>d, this cannot go on for any time however short, no matter how feeble the current may be, otherwise more energy would be evolved in the cell than is absorbed in the battery. If we insert an electrolytic cell containing dilute sulphuric acid along with a galvanometer into a circuit in which there is a single cell of Darnell, we observe the galvano meter needle swing out vigorously, and then settle down to a small and gradually decreasing deflection. The current ultimately becomes zero ; 2 but the time it takes to do so may be considerable, and varies with the nature of the electrodes. If we remove the battery after the current has stopped, and connect the polarized cell with the galvano meter, we observe an initial swing very nearly equal to the former but in the opposite direction, and a corresponding deflection, which after a time disappears entirely. Although, as a rule, a sensible time elapses before the polarization reaches its maximum, yet it is important to remark that it may rise to a very considerable fraction of the maximum in a very short* time indeed. Ecllund 3 found that in a cer tain case the electromotive force of polarization reached - 57 of a Daniell in about -5 of a second. Bernstein has recently arrived at results of a similar kind. He found, for instance, that platinum plates, polarized to 1 85 of a Daniell, fell, when the resistance of the circuit was 7 "4 6 Siemens units, to T57 in -00111 sec. 4 This rapidity of the rise and fall of the polarization is of very great importance, and has, we think, been overlooked by some experimenters. In cases where the polarization does not reach its maxi mum, no liberation of gas or other ion is observed, such as is seen with a permanent current, and it might of course be denied that chemical decomposition takes place at all. We shall, however, assume that Faraday s law holds for this case also, and assert that the current in the first instance actually passes through the liquid and produces chemical decomposition, according to the same law as a permanent current, and that this goes on until the accumu- 1 Maxwell, Electricity, vol. i. 266. J For an exception to this statement see Vloxr, p. 87.
- Pofjg. Ann., Ixxxv., 1852. * Foyy. Ann., civ . IS7f>.
