Oct. 20, 1887] 



NATURE 



583 



measurement. It is called its "specific conductivity" or 

 its "specific resistance" according to the way it is 

 regarded. The law here stated is called Ohm's law, and 

 and is one of the most accurately-known laws there are. 

 Nevertheless it is an empirical relation ; in other words, 

 it has not yet been accounted for — it must be accepted as 

 an experimental fact. Undoubtedly, it is one of vast and 

 far-reaching importance : it asserts a connexion between 

 electricity and ordinary matter of a definite and simple 

 kind. 



Now if we think of this opposing electromotive force as 

 analogous to friction, it is very easy to think of heat being 

 generated by the passage of a current, and to suppose 

 that the rate of heat-production will be directly propor- 

 tional to the opposing force and to the current driven 

 against it— as in fact Joule experimentally proved it 

 to be. 



But if we are not satisfied with this vague analogy, and 

 wish to penetrate into the ultimate nature of heat and the 

 mode in which it can be generated, then we can return to 

 the consideration of a multitude of oscillating and collid- 

 ing particles moving with a cettain average energy which 

 ■determines what we call the temperature of the body. If 

 now one or more of these bodies receives a knock, the 

 ■energy of the blow is speedily shared among all the 

 others, and they all begin to move rather more ener- 

 getically than before : the body which the assemblage of 

 particles constitutes is said to have risen in temperature. 

 This illustrates the production of heat by a blow or other 

 -mechanical means. But now, instead of striking one of 

 the balls, give it an electric charge ; or, better still, put 

 within its reach a constant reservoir of electricity from 

 which it can receive a charge every time it strikes it, and 

 at the same time put within the reach of some other of 

 the assemblage of particles another reservoir of infinite 

 capacity which shall be able to drain away all the elec- 

 tricity it may receive. In practice there is no need of 

 infinite reservoirs : all that is wanted is to connect two 

 finite reservoirs, or "electrodes," as one might now call 

 them, with some constant means of propelling electricity 

 from one to the other, i.e. with the poles of a voltaic 

 battery or a Holtz machine. 



What will be the result of thus passing a series of 

 ■electric charges through the assemblage of particles ? 

 Plainly the act of receiving a charge and passing it on 

 will tend to increase the original motion of each particle ; 

 it will tend to raise the temperature of the body. In this 

 w^ay, therefore, it is possible to picture the mode in which 

 an electric current generates heat. 



But although this process may be used as a possible 

 -analogy, it cannot be a true and complete statement of 

 what occurs ; for it is essentially the mode of propaga- 

 tion of sound. Sound travels at a definite and known 

 velocity, being a mechanical disturbance handed on from 

 particle to particle in the manner described. But heat, 

 being some mode of motion, must also be handed on 

 after some analogous fashion, so that when heat is sup- 

 plied to one point of a mass it spreads or diffuses through 

 it. It is difficult to suppose the conduction of heat to be 

 -other than the handing on of molecular quiverings from 

 one to another, and yet it takes place according to laws 

 altogether different from those of the propagation of the 

 gross disturbance called sound. The exact mode of con- 

 duction of heat is unknown, but, whatever it is, it can 

 hardly be doubted that the conduction of electricity 

 through metals is not very unlike it, for the two processes 

 obey the same laws of propagation : they are both of the 

 nature of a diffusion, they both obey Ohm's law, and a 

 metal which conducts heat well conducts electricity well 

 also. 



Conduction in Liquids. 



Leaving the obscure subject of conduction in metals 

 for the present, let us pass to the consideration of the 



way in which electricity flows through liquids. By 

 " liquids," in the present connexion, one more particu- 

 larly means definite chemical compounds, such as acids, 

 alkalies, salt and water, and saline solutions generally. 

 Some liquids there are, like alcohol, turpentine, bisulphide 

 of carbon, and possibly water, which, when quite pure 

 either wholly or very nearly decline to conduct electricity 

 at all. Such liquids as these may be classed along with 

 air and gases as more or less perfect dielectrics. Other 

 liquids there are, like mercury and molten metals 

 generally, which conduct after precisely the same fashion 

 as they do when solid. These therefore are properly 

 classed among metallic conductors. 



But most chemical compounds, when liquefied either 

 by heat or by solution, conduct in a way peculiarly their 

 own ; and these are called " electrolytes." 



The present state of our knowledge enables us to make 

 the following assertions with considerable confidence of 

 their truth : — 



(i) Electrolytic conduction is invariably accompanied 

 by chemical decomposition, and in fact only occurs by 

 means of it. 



(2) The electricity does not flow through, but with, the 

 atoms of matter, which travel along and convey their 

 charges something after the manner of pith balls, 



(3) The electric charge belonging tp each atom of 

 matter is a simple multiple of a definite quantity of elec- 

 tricity, which quantity is an absolute constant quite inde- 

 pendent of the nature of the particular substance to which 

 the atoms belong. 



(4) Positive electricity is conveyed through a liquid by 

 something equivalent to a procession of the electro- 

 positive atoms of the compound in the direction called 

 the direction of the current ; and at the same time nega- 

 tive electricity is conveyed in the opposite direction by a 

 similar procession of the electro-negative atoms. 



(5) On any atom reaching an electrode it may be forced 

 to get rid of its electric charge, and, combining with 

 others of the same kind, escape in the free state : in which 

 case visible decomposition results. Or it may find some- 

 thing else handy with which to combine — say on the 

 electrode or in the solution ; and in that case the decom- 

 position, though real, is masked, and not apparent. 



(6) But, on the other hand, the atom may cling to its 

 electric charge with such tenacity as to stop the current : 

 the opposition force exerted by these atoms upon the 

 current being called polarization. 



(7) No such opposition force, or tending to spring back, 

 is experienced in the interior of a mass of fluid : it occurs 

 only at the electrodes. 



It would take too long to go into the evidence for these 

 statements and to adduce examples : I will try and make 

 the process of electrolytic conduction clearer by reverting 

 to our mechanical analogies and models. 



Looking back to Figs. 5 and 6 (p. 559), we see illustrations 

 of metallic conduction and of dielectric induction. In each 

 case an applied electromotive force causes some move- 

 ment of electricity ; but, whereas in the first case it is a 

 continuous almost unresisted movement or steady flow 

 through or among the atoms of matter, in the second 

 case it is a momentary shift or displacement only, carry- 

 ing the atoms of matter with it, and highly resisted in 

 consequence : — resisted, not with a mere frictional rub 

 which retards but does not check the motion, but by an 

 active spring back force, which immediately checks all 

 further current, produces what we call "insulation," and 

 ultimately, when the propelling force is removed, causes a 

 quick reverse motion or discharge. But the model is 

 plainly an incomplete one: for what is it that the atoms 

 are clinging to ? What is it ought to take the place of the 

 beam in the crude mechanical contrivance ? Obviously 

 another set of atoms, which are either kept still or urged in 

 the opposite direction by a simultaneous opposite displace- 

 ment of negative electricity. We are to picture two or 



