Page:1902 Encyclopædia Britannica - Volume 26 - AUS-CHI.pdf/796

This page needs to be proofread.


740

CHEMISTRY

that this would happen in liquids generally, although to different extents. Nor is it clear why the disruption should be of so partial a character, and lead only to the production of ions in the case of complex molecules; for example, acetic acid, ex hypothesi, is resolved only into H and C2H302, instead of the molecule being more or less shattered to pieces. From this point of view it is impossible also to understand why only protochlorides and not perchlorides are electrolytes. The discovery that chemical interchanges are in many cases not of the form AB + CD = AC + BD, but require the intervention of a third substance—the presence of an electrolyte being a sine qua non—and that this is probably equally true of decompositions and isodynamic changes, indeed of every case of change, is almost conclusive evidence against the acceptance of the dissociation hypothesis either in the form suggested by Williamson or in that suggested by Clausius; in fact, no satisfactory evidence has been adduced that molecules are per se intrinsically so unstable as is suggested, or that they are in the habit of colliding with such violence as to smash each other to pieces. As solvents generally do not condition electrolysis, but only particular solvents, it must be supposed that the Electro solvent exercises a definite influence, and one lylts a° which is not of a merely mechanical order1; and dependent that the molecules of solvent and solute in some process. -way influence each other, and interact so as to pull each other to pieces as it were—in other words, that electrolysis is a dependent process. It is not a little remarkable that Faraday’s prescience guided him to such a conclusion as the following passage (§ 523) in his fifth series of researches shows :— The theory which I have ventured to put forth (almost) requires an admission that in a compound body capable of electrochemical decomposition the elementary particles have a mutual relation to and influence upon each other, extending beyond those with which they are immediately combined. Thus, in water, a particle of hydrogen in combination with oxygen is considered as not altogether indifferent to other particles of oxygen, although they are combined with other particles of hydrogen ; but to have an affinity of attraction towards them which, though it does not at all approach in force under ordinary circumstances to that by which it is combined with its own particle, can, under the electric influence exerted in a definite direction, be made even to surpass it. This general relation of particles already in combination to other particles with which they are not combined is sufficiently distinct in numerous results of a purely chemical character . . . and it probably has a direct relation to and connexion with attraction of aggregation both in solids and fluids. It is a remarkable circumstance that in gases and vapours, where the attraction of aggregation ceases, there likewise the decomposing powers of electricity apparently cease, and there also the chemical action of quantity is no longer evident. It seems not unlikely that the inability to suffer decomposition in these cases may be dependent upon the absence of that mutual attractive relation of the particles which is the cause of aggregation. In fact, when the phenomena generally of chemistry— and not merely the isolated behaviour of electrolytes—are taken into account, there can be little doubt that chemical changes are primarily associative and not dissociative in character, as was seen by Faraday; and that the force at work by which they are initiated is in all probability that which has been termed residual affinity. That ffonhypo- tllis has not been ni0re co.mmonly recognized thesis of probably arises from the circumstance that it chemical has been customary as a rule only to consider change. The tpe enq results of chemical operations. view taken by Kekule as to the manner in which chemical change is brought about needs but slight alteration to be in 1 It has been contended of late that the superior influence of water is due to its high specific inductive capacity. It is more probable that the power of water to condition electrolysis and its high specific inductive capacity are but correlated properties which have their origin in the high residual affinity of the oxygen.

harmony with such an argument; it is merely necessary to assume that when the complex formed by the association of the interacting or potentially interacting substances meets with the necessary third component, a conducting system is established, and that so soon as this is formed the change sets in. Residual affinities are in all probability the polarizing forces at work in such cases, by which the various components are led to associate and take up compatible positions. This hypothesis involves the assumption that complication, not simplification, precedes chemical change ; that complex molecular systems are first formed from the interacting substances, and that these, on breaking down, suffer rearrangement of the parts—such rearrangement taking place in consequence of elements which were previously separate being brought into one common “ sphere of activity ” within which it is possible for them to interact. The function of condensing and other agents—of catalysts generally—on this hypothesis is that of collecting into one system the various elements necessary for the occurrence of a particular change. Such systems may be regarded as, and in fact would be, closed voltaic circuits. The action of aluminium chloride, for example, in conditioning the formation of homologues of benzene from the two substances benzene and methyl chloride—both undoubtedly dielectrics, which when together alone are entirely without action on each other— may be ascribed to the formation of an unstable compound of both with the metallic chloride. In like manner ferric chloride probably conditions the interaction of bromine and benzene by combining with both, and so bringing them within each other’s range in an unstable system. Ordinary cases of chemical change differ from those effected by an electric current merely in being cases of exothermic change which take place without any energy being supplied from without. How the current acts in promoting disruption cannot apparently at present be determined, but it is highly probable that it circulates within complex systems such as have been referred to. A rasping contact may be said to exist between the molecules of the dissolved substance and the solvent, and the action of the current upon each complex may be roughly compared with that of a dredging machine scraping mud from the bottom of a stream. A number of such dredgers arranged in series, each one of which picks up the mud delivered by the machine below, and thus carries it a stage higher up, so that mud only is visibly delivered at the one end of the system, may be taken as a simile of the manner in which the transport of the negative ion is effected; the water taken up in the buckets, but returned into the stream down which it flows, corresponds to the positive ion. But although both are concerned in the process, it cannot at present be determined whether the water or the dissolved substance in an aqueous solution is primarily split up ; it is doubtful whether either is ever at once separately resolved into its ions. Probably in the first instance the system is resolved into two parts, one of which escapes—unless it act on the water—• while the other being unstable often gives rise to secondary products. A solution of sulphuric acid, for example, appears to yield hydrogen and “ oxidized sulphuric acid ”; the extent to which this latter persists depends entirely on the conditions, i.e., the concentration, the temperature, and the size and character. of the electrode. That water should be the most active among liquids both as a solvent and in promoting electrolysis is in no way surprising, as no other compound exhibits residual affinity in so high a degree. To account for the behaviour of conducting chlorides, it is only necessary to suppose that individual like molecules exercise among themselves an influence similar to that exercised in