538 



NATURE 



{April T, 1 88 1 



great , 'number of liquids, by Prof. Hittorff, of Miinster, 

 and Prof. G. Wiedemann, of Leipsic. 



Prof. F. Kohlrausch, of Wiirzburg, has brought to light 

 the very important fact that in diluted solutions of salts, 

 including hydrates of acids and hydrates of caustic 

 alkalis, every atom under the influence of currents of the 

 same density moves on with its own peculiar velocity, 

 independently of other atoms moving at the same time 

 in the same or in opposite directions. The total amount 

 of chemical motion in every section of the fluid is repre- 

 sented by the sum of the eciuivalents of the cation gone 

 forwards and of the anion gone backwards, in the same way 

 as in thedualistic theory of electricity, and the total amount 

 of electricity flowing through a section of the conductor 

 corresponds to the sum of positive electricity going 

 forwards and negative electricity going backwards. 



This established, Faraday's law tells us that through 

 each section of an electrolytic conductor we have always 

 equivalent electrical and chemical motion. The same 

 definite quantity of either positive or negative electricity 

 moves always with each univalent ion, or with every unit 

 of affinity of a multivalent ion, and accompanies it during 

 all its motions through the interior of the electrolytic 

 fluid. This we may call the electric charge of the atom. 



Now the most startling result, perhaps, of Faraday's 

 law is this : If we accept the hypothesis that the ele- 

 mentary substances are composed of atoms we cannot 

 avoid concluding that electricity also, positive as well 

 as negative, is divided into definite elementary por- 

 tions, which behave like atoms of electricity. As long as 

 it moves about on the electrolytic liquid each atom 

 remains united with its electric equivalent or equivalents. 

 At the surface of the electrodes decomposition can take 

 place if there is sufficient electromotive power, and then 

 the atoms give off their electric charges and become 

 electrically neutral. 



Now arises the question, Are all these relations between 

 electricity and chemical combination limited to that class 

 of bodies which we know as electrolytes ? In order to 

 produce a current of sufficient strength to collect enough 

 of the products of decomposition without producing too 

 much heat in the electrolyte, the substance which we try 

 to decompose ought not to have too much resistance 

 against the current. But this resistance may be very 

 great, and the motion of the ions may be very slow, so 

 slow indeed that we should need to allow it to go on for 

 hundreds of years before we should be able to collect 

 even traces of the products of decomposition ; neverthe- 

 less all the essential attributes of the process of electro- 

 lysis could subsist. If you connect an electrified 

 conductor with one of the electrodes of a cell filled with 

 oil of turpentine, the other with the earth, you will find 

 that the electricity of the conductor is discharged unmis- 

 takably more rapidly through the oil of turpentine than if 

 you take it away and fill the cell only with air. 



Also in this case we may observe polarisation of the 

 electrodes as a symptom of previous electrolysis. An- 

 other sign of electrolytic conduction is that liquids 

 brought between two different metals produce an electro- 

 motive force. This is never done by metals of equal 

 temperature, or other conductors which, like metals, let 

 electricity pass without being decomposed. 



The same effect is also observed even with a great 

 many rigid bodies, although we have very few solid bodies 

 which allow us to observe this electrolytic conduction with 

 the galvanometer, and even these only at temperatures 

 near to their melting-point. It is nearly impossible to 

 shelter the quadrants of a delicate electrometer against 

 being charged by the insulating bodies by which they are 

 supported. 



In all the cases which I have quoted one might suspect 

 that traces of humidity absorbed by the substance or 

 adhering to their surface were the electrolytes. I show 

 you therefore this little Danicll's cell, in which the porous 



septum has been substituted by a thin stratum of glass. 

 Externally all is symmetrical at both poles; there is 

 nothing in contact with the air but a closed surface of 

 glass, through which two wires of platinum penetrate. 

 The whole charges the electrometer exactly like a 

 Daniell's cell of very great resistance, and this it would 

 not do if the septum of glass did not behave like an 

 electrolyte. All these facts show that electrolytic con- 

 duction is not at all limited to solutions of acids or salts. 



Hitherto we have studied the motions of ponderable 

 matter as well as of electricity, going on in an electrolyte. 

 Let us study now the forces which are able to produce 

 these motions. It has always appeared somewhat start- 

 ling to everybody who knows the mighty power of che- 

 mical forces, the enormous quantity of heat and of 

 mechanical work which they are able to produce, and 

 who compares with it the exceedingly small electric 

 attraction which the poles of a battery of two Daniell's 

 cells show. Nevertheless this little apparatus is able to 

 decompose water. 



The quantity of electricity which can be conveyed by a 

 very small qumtity of hydrogen, when measured by its 

 electrostatic forces, is exceedingly great. Faraday saw 

 this, and has endeavoured in various ways to give at 

 least an approximate determination. The most powerful 

 batteries of Leyden jars, discharged through a voltameter, 

 give scarcely any visible traces of gases. At present we 

 can give definite numbers. The result is that the electri- 

 city of I mgrm. of water, separated and communicated to 

 two balls, I kilometre distant, would produce an attraction 

 between them, equal to the weight of 25,000 kilos. 



The total force exerted by the attraction of an electrified 

 body upon another charged with opposite electricity is 

 always proportional to the quantity of electricity con- 

 tained in the attracting as on the attracted body, and 

 therefore even the feeble electric tension of two Daniell's 

 elements acting through an electrolytic cell upon the 

 enormous quantities of electricity with which the con- 

 stituent ions of water are charged, is mighty enough to 

 separate these elements and to keep them separated. 



We now turn to investigate what motions of the ponder- 

 able molecules require the action of these forces. Let us 

 begin with the case where the conducting liquid is sur- 

 rounded everywhere by insulating bodies. Then no 

 electricity can enter, none can go out through its surface, 

 but positive electricity can be driven to one side, negative 

 to the other, by the attracting and repelling forces of 

 external electrified bodies. This process going on as 

 well in every metallic conductor is called " electrostatic 

 induction." Liquid conductors behave quite like metals 

 under these conditions. Prof Wtillner has proved that even 

 our best insulators, exposed to electric forces for a long 

 time, are charged at last quite in the same way as metals 

 would be charged in an instant. There can be no doubt 

 that even electromotive forces going down to less than 

 iTTj Daniell produce perfect electrical equilibrium in the 

 interior of an electrolytic liquid. 



Another somewhat modified instance of the same 

 effects is afforded by a voltametric cell containing two 

 electrodes of platinum, which are connected with a 

 Daniell's cell, the electromotive force of which is insuf- 

 ficient to decompose the electrolyte. Under this condi- 

 tion the ions carried to the electrodes cannot give oft' 

 their electric charges. The whole apparatus behaves, as 

 was first accentuated by Sir W. Thomson, like a con- 

 denser of enormous capacity. 



Observing the polarising and depolarising currents in 

 a cell containing two electrodes of platinum, hermetically 

 sealed and freed of all air, we can observe these pheno- 

 mena with the most feeble electromotive forces of -^-^t, 

 Daniell, and I found that down to this limit the capacity 

 of the platinum surfaces proved to be constant. By 

 taking greater surfaces of platinum I suppose it will be 

 possible to reach a limit much lower than that. If any 



