Text: When he began his experiments, neither Daniell's nor Grove's battery was known, and there were no means of producing currents of constant intensity; the methods of measuring this intensity were also in their infancy. This may excuse his predecessors. Faraday overcame this difficulty by sending the same current of electricity for the same time through a series of two or more electrolytic cells. He proved at first that the dimensions of the cell and the size of the metallic plates through which the current entered and left the cell had no visible influence upon the quantity of the products of decomposition. Cells containing the same electrolytic fluid between plates of the same metals gave always the same quantity after being traversed by the same current. Then he compared the amount of decomposition in cells containing different electrolytes, and he found it exactly proportional to the chemical equivalents of the elements, which were either separated or converted into new compounds. Faraday concluded from his experiments that a definite quantity of electricity cannot pass a voltametric cell containing acidulated water between electrodes of platinum without setting free at the negative electrode a corresponding definite amount of hydrogen and at the positive electrode the equivalent quantity of oxygen, one atom of oxygen for every pair of atoms of hydrogen. If, instead of hydrogen, any other element capable of replacing hydrogen is separated from the electrolyte, this is done also in a quantity exactly equivalent to the quantity of hydrogen which would have been evolved by the same electric current. According to the modern chemical theory of quantivalence, therefore, the same quantity of electricity passing through an electrolyte either sets free or transfers to other combinations always the same number of units of affinity at both electrodes; for instance, instead H K Na of H, either K, or Na, or Ba}, or Ca}, or Zn}, or Cu} from cupric salts, Cu or Cu from cuprous salts, and so on. The simple or compound halogens separating at the other electrodes are equivalent, of course, to the quantity of the metallic element with which they were formerly combined. According to Berzelius' theoretical views, the quantity of electricity collected at the point of union of two atoms ought to increase with the strength of their affinity. Faraday demonstrated by experiment that, insofar as this electricity came forth in electrolytic decomposition, its quantity did not at all depend on the degree of affinity. This was really a fatal blow to Berzelius' theory. Since that time our experimental methods and our knowledge of the laws of electrical phenomena have made enormous progress, and a great many obstacles have now been removed which entangled every one of Faraday's steps and obliged him to fight with the confused ideas and ill-applied theoretical conceptions of some of his contemporaries. The original voltameter of Faraday, an instrument which measured the quantity of gases evolved by the decomposition of water in order to determine with it the intensity of the galvanic current, has been replaced by the silver voltameter of Poggendorff, which permits of much more exact determinations by the quantity of silver deposited from a solution of silver nitrate on a strip of platinum. We have galvanometers which not only indicate that there is a galvanic current but likewise measure its electromagnetic intensity very exactly and in a very short time, and do this as well for the highest as for the lowest degrees of intensity. We have electrometers, like the quadrant electrometer of Sir W. Thomson, able to measure differences of electric potential corresponding to less than one hundredth of a Daniell cell. As for the frequently used term electric potential, a term introduced by Green, you may translate it as signifying the electric pressure to which the positive unit of electricity is subject at a certain place. We need not hesitate to say that the more experimental methods were refined, the more completely were confirmed the exactness and generality of Faraday's law. In the beginning Berzelius and the adherents of Volta's original theory of galvanism, based on the effects of metallic contact raised many objections against Faraday's law. By the combination of Nobili's astatic pairs of magnetic needles with Schweigger's multiplier, a coil of copper wire with numerous circumvolutions, galvanometers became so delicate that they were able to indicate the electrochemical equivalent of currents so feeble as to be quite imperceptible by all chemical methods. With the newest galvanometers you can very well observe currents which would have to last a century before decomposing one milligram of water, the smallest quantity that is usually weighed on chemical balances. You see that if such a current lasts only some seconds or some minutes, there is not the slightest hope of discovering its products of decomposition by chemical analysis. And even if it should last a long time, the minute quantities of hydrogen collected at the negative electrode may vanish because they combine with the traces of atmospheric oxygen absorbed by the liquid. Under such conditions a feeble current may continue as long as you like without producing any visible trace of electrolysis, not even of galvanic polarization, the appearance of which can be used as an indication of previous electrolysis. Galvanic polarization, as you know, is an altered stated of the metallic plates which have been used as electrodes during the decomposition of an electrolyte. Polarized electrodes, when connected by a galvanometer, give a current which they did not give before being polarized. By this current the plates are discharged again and returned to their original state of equality. The most probable explanation of this polarization is that molecules of the electrolyte, charged with electricity, are carried by the current to the surface of the metal, itself charged with opposite electricity, and are retained there by electric attraction. That really constituent atoms of the electrolyte partake in the production of galvanic polarization cannot well be doubted, because this state can be produced and also destroyed purely by chemical mean. If hydrogen has been carried to an electrode by the current, contact with the atmospheric oxygen removes the state of polarization. The depolarizing current is indeed a most delicate means of discovering previous decomposition. But even this may fail if the nascent polarization is destroyed by an intervening chemical action, like that of the oxygen of the air. To avoid this, delicate experiments on this subject cannot be performed except in vessels carefully purified of all gases. I have lately succeeded in doing this in a far more perfect way than before by using a hermetically sealed cell (Fig. 1) which contains water acidulated with sulfuric acid. Two platinum wires, b and c, and a third platinum wire, a, which in the interior is connected with a spiral of palladium, can be used as electrodes. The tube, before it was closed, had been connected with an air-water pump, and at the same time oxygen was evolved from b and c by tow Grove's elements; the hydrogen carried to the palladium wire, a, was occluded in the metal. In this way the liquid in the tube is washed out with oxygen under low pressure and freed from all other gases. After the closing of the tube, the remaining small traces of electrolytic oxygen combine slowly with the hydrogen of the palladium. Traces of hydrogen occluded in the platinum wires b and c can be transferred by a feeble electromotive force into the palladium; and even new quantities of electrolytic gases, evolved after closing the tube, can be removed again by a Daniell cell, which carries hydrogen to the palladium, where it is occluded, and oxygen to b and c, where it combines with hydrogen, as long as traces of this gas are dissolved in the liquid. The rest of the oxygen absorbed by the liquid combines with the occluded hydrogen. I have ascertained with this apparatus that under favorable conditions one can observe the polarization produced during a few seconds by a current which would decompose only one milligram of water in a century. But even if the appearance of galvanic polarization should not be acknowledged by opponents as a sufficient indication of previous decomposition, it is not difficult at present to reduce the indications of a good galvanometer to absolute measure, to calculate the amount of decomposition which ought to be expected according to Faraday's law, and to verify that in all the cases in which no products of electrolysis can be discovered, their amount is too small for chemical analysis. Products of decomposition cannot appear at the electrodes without the constituent molecules of the electrolyte throughout the whole length of the liquid. On this point the majority of Faraday's predecessors were already agreed, but they differed from one another as soon as they came to the question what those notions were. Faraday saw very clearly the importance of this problem and again appealed to experiment. He filled two cells with an electrolytic fluid, connecting them by a thread of asbestos wetted with the same fluid, in order to determine separately the quantity of all the chemical constituents transferred to the one and the other extremity of the electrolytic conductor. You know that he proposed for these atoms or groups of atoms transported by the current through the fluid the Greek word ions ("the travelers"); and comparing the current of positive electricity with a stream of water, he called cations those atoms which go down the stream in the same direction with the positive electricity to the cathode, the metallic plate through which this electricity left the fluid. The anions, on the contrary, go up the stream to the anode, the metal plate which is the source of the current +E. Cations generally are atoms which are substitutes of hydrogen; anions are halogens.

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