584 



NA TURE 



{Oct. 20, 1887 



any number of rows of beads, each row threaded on its 

 appropriate cord ; the cords alternately representing 

 positive and negative electricity respectively, and being 

 simultaneously displaced in opposite directions by any 

 applied E.M.F. The beads threaded on any one cord 

 have, in a dielectric, elastic attachments to those on some 

 opposite cord, and thus continuous motion of the cords 

 in opposite directions is prevented : only a slight displace- 

 ment is permitted, followed by a spring back and oscilla- 

 tion after the fashion already described. 



Very well ; now picture the elastic connexions between 

 the beads all dissolved, and once more apply a force to 

 each cord, moving half of them one way and the alter- 

 nate half the other way, and you have a model illustrating 

 an electrolyte and electrolytic conduction. The atoms 

 are no longer attached to each other, but they are 

 attached to the cord. In the first respect, an electrolyte 

 differs from a dielectric ; in the second, it differs from 

 a metal. 



Moreover, electrolytic conduction is perceived to be 

 scarcely of the nature of true conduction : the electricity 

 does not slip through or among the molecules, it goes 

 with them. The constituents of each molecule are free of 

 each other, and while one set of atoms conveys positive 



electricity, the other set carries negative electricity in the 

 opposite direction ; and so it is by a procession of free 

 atoms that the current is transmitted. The process is 

 of the nature of convection : the atoms act as carriers. 

 Free locomotion of charged atoms is essential to electro- 

 lysis. 



In order to compare with Figs. 5 and 6, so as to bring 

 out the points of difference, Fig. 13 is drawn. The beads 

 representing one set of atoms of matter are tightly at- 

 tached to the cord, no trace of slip between them being 

 permitted, but otherwise they are free, and so are repre- 

 sented as supported merely by rings sliding freely on 

 glass rods. The only resistance to the motion, beside 

 the slight friction, is offered at the electrode, which is 

 typified by the spring-backed knife-edge, z. This is sup- 

 posed to be able to release the beads from the cord when 

 they are pressed against it with sufficient force. The 

 cling between the bead and cord (z.<?. between each 

 atom and its charge) is great enough to cause a percept- 

 ible compression of the springs, and accordingly to bring 

 out a recoil force in imitation of polarization. 



The piece of cord accompanying each bead on its 

 journey {i.e. the length between it and the next bead) 

 represents the atomic charge, and is a perfectly con- 



FiG. i3.^Crude mechanical analogy, illustrating a few points in a circuit partly electrolytic. 



stant quantity : the only variation permissible in it is 

 that some kinds of atoms have twice as much, or are 

 twice as far apart on their cord, and these are called 

 by chemists dyad atoms ; another kind- has three times 

 as much, another four, and so on ; these being called triad, 

 tetrad, &c. 



If the coi'd be taken to represent positive electricity, 

 the beads on it may represent atoms of hydrogen, or 

 other monad cation, travelling down stream to the 

 cathode. Another cord representing negative electricity 

 may be ranged alongside it, with its beads twice as far 

 apart, to represent the atoms of a dyad anion, like oxy- 

 gen. If the cords are so mechanically connected that 

 they must move with equal pace in opposite directions, 

 we have a model illustrating several important facts. 

 The number of oxygen atoms liberated in a given time 

 will be obviously half the number of hydrogen atoms set 

 free in the same time, and will therefore in the gaseous 

 state occupy but half the volume. For any element 

 whatever, the number of atoms liberated in any time is 

 equal to the number of atoms of hydrogen liberated in 

 the same time, divided by the "valency" of the element 

 as compared with hydrogen. This law was discovered 

 by Faraday, and appears to be precisely true ; and inas- 



much as the relative weight of every element is known 

 with fair accuracy, it is easy to calculate what weight of 

 substance any given current will deposit or set free in an 

 hour, if we once determine it experimentally for any one 

 substance. 



We may summarize thus : — 



If we apply E.M.F. to a metal we get a continuous 

 flow, and the result is heat. 



If we apply it to a dielectric we get a momentary flow 

 or displacement, and the result is the potential energy of^ 

 '•' charge." 



If we apply it to an electrolyte we again get a con- 

 tinuous flow, and the result is chemical decomposition. 



There are a large number of important points to which| 

 I might direct your attention in the mode by which ar 

 electric current is conveyed through liquids, but I vvil 

 specially select one, viz. that it is effected by a procession' 

 of positively charged atoms travelling one way, and a 

 corresponding procession of negatively charged atoms ™| 

 the other way. ■! 



Whatever we understand by a positive charge and a^i 

 negative charge, it is certain that the atoms of, say a 

 water molecule, are charged, the hydrogen positively, the 

 oxygen negatively ; and it is almost certain that they j 



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