Part 24 (2/2)

826. i. A single _ion_, i.e. one not in combination with another, will have no tendency to pa.s.s to either of the electrodes, and will be perfectly indifferent to the pa.s.sing current, unless it be itself a compound of more elementary _ions_, and so subject to actual decomposition. Upon this fact is founded much of the proof adduced in favour of the new theory of electro-chemical decomposition, which I put forth in a former series of these Researches (518. &c.).

827. ii. If one _ion_ be combined in right proportions (697.) with another strongly opposed to it in its ordinary chemical relations, i.e. if an _anion_ be combined with a _cation_, then both will travel, the one to the _anode_, the other to the _cathode_, of the decomposing body (530, 542.

547.).

828. iii. If, therefore, an _ion_ pa.s.s towards one of the electrodes, another _ion_ must also be pa.s.sing simultaneously to the other electrode, although, from secondary action, it may not make its appearance (743.).

829. iv. A body decomposable directly by the electric current, i.e. an _electrolyte_, must consist of two _ions_, and must also render them up during the act of decomposition.

830. v. There is but one _electrolyte_ composed of the same two elementary _ions_; at least such appears to be the fact (697.), dependent upon a law, that _only single electro-chemical equivalents of elementary ions can go to the electrodes, and not multiples_.

831. vi. A body not decomposable when alone, as boracic acid, is not directly decomposable by the electric current when in combination (780.).

It may act as an _ion_ going wholly to the _anode_ or _cathode_, but does not yield up its elements, except occasionally by a secondary action.

Perhaps it is superfluous for me to point out that this proposition has _no relation_ to such cases as that of water, which, by the presence of other bodies, is rendered a better conductor of electricity, and _therefore_ is more freely decomposed.

832. vii. The nature of the substance of which the electrode is formed, provided it be a conductor, causes no difference in the electro-decomposition, either in kind or degree (807. 813.): but it seriously influences, by secondary action (714.), the state in which the finally appear. Advantage may be taken of this principle in combining and _ions_ collecting such _ions_ as, if evolved in their _free_ state, would be unmanageable[A].

[A] It will often happen that the electrodes used may be of such a nature as, with the fluid in which they are immersed, to produce an electric current, either according with or opposing that of the voltaic arrangement used, and in this way, or by direct chemical action, may sadly disturb the results. Still, in the midst of all these confusing effects, the electric current, which actually pa.s.ses in any direction through the body suffering decomposition, will produce its own definite electrolytic action.

833. viii. A substance which, being used as the electrode, can combine with the _ion_ evolved against it, is also, I believe, an _ion_, and combines, in such cases, in the quant.i.ty represented by its _electro-chemical equivalent_. All the experiments I have made agree with this view; and it seems to me, at present, to result as a necessary consequence. Whether, in the secondary actions that take place, where the _ion_ acts, not upon the matter of the electrode, but on that which is around it in the liquid (744.), the same consequence follows, will require more extended investigation to determine.

834. ix. Compound _ions_ are not necessarily composed of electro-chemical equivalents of simple _ions_. For instance, sulphuric acid, boracic acid, phosphoric acid, are _ions_, but not _electrolytes_, i.e. not composed of electro-chemical equivalents of simple _ions_.

835. x. Electro-chemical equivalents are always consistent; i.e. the same number which represents the equivalent of a substance A when it is separating from a substance B, will also represent A when separating from a third substance C. Thus, 8 is the electro-chemical equivalent of oxygen, whether separating from hydrogen, or tin, or lead; and 103.5 is the electrochemical equivalent of lead, whether separating from oxygen, or chlorine, or iodine.

836. xi. Electro-chemical equivalents coincide, and are the same, with ordinary chemical equivalents.

837. By means of experiment and the preceding propositions, a knowledge of _ions_ and their electro-chemical equivalents may be obtained in various ways.

838. In the first place, they may be determined directly, as has been done with hydrogen, oxygen, lead, and tin, in the numerous experiments already quoted.

839. In the next place, from propositions ii. and iii., may be deduced the knowledge of many other _ions_, and also their equivalents. When chloride of lead was decomposed, platina being used for both electrodes (395.), there could remain no more doubt that chlorine was pa.s.sing to the _anode_, although it combined with the platina there, than when the positive electrode, being of plumbago (794.), allowed its evolution in the free state; neither could there, in either case, remain any doubt that for every 103.5 parts of lead evolved at the _cathode_, 36 parts of chlorine were evolved at the _anode_, for the remaining chloride of lead was unchanged.

So also, when in a metallic solution one volume of oxygen, or a secondary compound containing that proportion, appeared at the _anode_, no doubt could arise that hydrogen, equivalent to two volumes, had been determined to the _cathode_, although, by a secondary action, it had been employed in reducing oxides of lead, copper, or other metals, to the metallic state. In this manner, then, we learn from the experiments already described in these Researches, that chlorine, iodine, bromine, fluorine, calcium, pota.s.sium, strontium, magnesium, manganese, &c., are _ions_ and that their _electro-chemical equivalents_ are the same as their _ordinary chemical equivalents_.

840. Propositions iv. and v. extend our means of gaining information. For if a body of known chemical composition is found to be decomposable, and the nature of the substance evolved as a primary or even a secondary result (743. 777.) at one of the electrodes, be ascertained, the electro-chemical equivalent of that body may be deduced from the known constant composition of the substance evolved. Thus, when fused protiodide of tin is decomposed by the voltaic current (804.), the conclusion may be drawn, that both the iodine and tin are _ions_, and that the proportions in which they combine in the fused compound express their electro-chemical equivalents. Again, with respect to the fused iodide of pota.s.sium (805.), it is an electrolyte; and the chemical equivalents will also be the electro-chemical equivalents.

841. If proposition viii. sustain extensive experimental investigation, then it will not only help to confirm the results obtained by the use of the other propositions, but will give abundant original information of its own.

842. In many instances, the _secondary results_ obtained by the action of the evolved _ion_ on the substances present in the surrounding liquid or solution, will give the electro-chemical equivalent. Thus, in the solution of acetate of lead, and, as far as I have gone, in other proto-salts subjected to the reducing action of the nascent hydrogen at the _cathode_, the metal precipitated has been in the same quant.i.ty as if it had been a primary product, (provided no free hydrogen escaped there,) and therefore gave accurately the number representing its electro-chemical equivalent.

843. Upon this principle it is that secondary results may occasionally be used as measurers of the volta-electric current (706. 740.); but there are not many metallic solutions that answer this purpose well: for unless the metal is easily precipitated, hydrogen will be evolved at the _cathode_ and vitiate the result. If a soluble peroxide is formed at the _anode_, or if the precipitated metal crystallize across the solution and touch the positive electrode, similar vitiated results are obtained. I expect to find in some salts, as the acetates of mercury and zinc, solutions favourable for this use.

844. After the first experimental investigations to establish the definite chemical action of electricity, I have not hesitated to apply the more strict results of chemical a.n.a.lysis to correct the numbers obtained as electrolytic results. This, it is evident, may be done in a great number of cases, without using too much liberty towards the due severity of scientific research. The series of numbers representing electro-chemical equivalents must, like those expressing the ordinary equivalents of chemically acting bodies, remain subject to the continual correction of experiment and sound reasoning.

845. I give the following brief Table of _ions_ and their electro-chemical equivalents, rather as a specimen of a first attempt than as anything that can supply the want which must very quickly be felt, of a full and complete tabular account of this cla.s.s of bodies. Looking forward to such a table as of extreme utility (if well-constructed) in developing the intimate relation of ordinary chemical affinity to electrical actions, and identifying the two, not to the imagination merely, but to the conviction of the senses and a sound judgement, I may be allowed to express a hope, that the endeavour will always be to make it a table of _real_, and not _hypothetical_, electro-chemical equivalents; for we shall else overrun the facts, and lose all sight and consciousness of the knowledge lying directly in our path.

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