246 Mr. W. C. Dampier Whetham [Feb. 16, 



but at the ends the parts are set free. The conception may be 

 illustrated by a model, in which differently coloured balls, fixed to 

 movable strings, represent the opposite parts of the salt. 



These moving parts of the salt must be electrified, since they 

 move when acted on by an electric force. They were called by 

 Faraday the ions. To a further study of the nature and properties of 

 these ions I ask your attention to-night. 



Faraday found that, on passing a steady electric current through 

 a decomposable liquid or electrolyte, the amount of chemical decom- 

 position was proportional to the strength of the electric current, and 

 to the time of current-flow — that is, to the total quantity of electricity 

 which passes. Hence a given quantity of any ion such as copper or 

 chlorine must carry with it a definite charge of electricity. More- 

 over, the mass of substance deposited by a given current in a given 

 time was found to be proportional to its chemical equivalent weight. 

 Thus equal numbers of equivalents, whether of the same or of 

 different ions, must be associated with equal charges. 



If we accept the atomic theory, we must regard the chemical 

 equivalent w^eight of a substance as proportional to the mass of its 

 atom divided by its valency, i.e. by the number of univalent atoms 

 such as that of hydrogen which one atom of the substance will com- 

 bine with or displace. Faraday's experiments then mean that each 

 univalent ion carries the same charge of electricity, each divalent 

 ion twice that charge, and so on. The charge on one univalent ion 

 is a true natural unit of electricity, which is thus seen to be quite as 

 atomic in its nature as is matter. 



We must now regard the process of electrolysis (i.e. the passage 

 of electricity through a decomposable liquid or electrolyte) as a kind 

 of convection, the electric current being carried through the liquid 

 somewhat as water may be carried from point to point in a number of 

 buckets. 



If a current be passed for some time through a solution such as 

 that of copper sulphate, not only is copper dissolved from the anode, 

 or plate by which the current is said conventionally to enter the 

 solution, and deposited at the cathode or plate of exit, but a notable 

 change in concentration is noted in the solution near the two 

 electrodes. The liquid near the anode becomes more concentrated, 

 and that near the cathode more dilute. This may easily be illustrated 

 by an experiment. If instead of copper we use platinum as anode it 

 is not dissolved, and the total amount of copper in solution pro- 

 gressively diminishes. We then find that, while salt is taken from 

 the neighbourhood of both electrodes, more comes from the cathode 

 than from the anode. These phenomena were studied extensively by 

 Hittorf about the years 1850-1860. 



Two explanations of this uneven dilution of the solution are 

 possible. We may suppose that the ions are complex structures, and 

 drag unaltered salt or solvent with them through the liquid, or we 



