Page:Text-book of Electrochemistry.djvu/129

 114 ELECTROLYTES. chap.

not altered, and a current can be obtained by using the smallest conceivable electromotive force. Even when work- ing with polarisable electrodes it is only in order to transport the electricity from the electrode to the solution, or vice versa, that a certain electromotive force is required. In this case also the smallest fall of potential is sufficient to cause a current in the liquid. This fact was proved by Buff (2) with currents so small that it was only after months that a cubic centimetre of explosive mixture was obtained.

According to this, the very smallest force is sufficient to split the molecules in the Grotthuss chain. The behaviour of gases indicates what the relationships would be if the mole- cules of electrolytes were undecomposed. In order to keep a constant electric current passing through a gas a certain fall of potential per centia\,etre is necessary, and this is dependent on the pressure of the gas.

Faraday's view is therefore incorrect. The radicles of the salt molecule cannot be held together by a force of finite value. This was first appreciated by Clausius (1857), who was led to the assumption that in a solution of an electrolyte a constant exchange of ions is taking place between the mole- cules, or that, in special exceptional cases, free ions occur. According to this view the electrolyte is " electrolytically dissociat€Kl" into its ions, although these are present in such small quantity as not to be recognisable by chemical tests. Clausius arrived at these conclusions by the following considerations : According to the kinetic theory heat is due to the rapid motion of the molecules, and on the average the kinetic energy of a gram-molecule of every kind of gas molecule is the same. This motion is very great ; at 20° the velocity of an oxygen molecule is 425 metre/sec, of a hydrogen molecule 1700 met./sec, and of a molecule of water vapour 566 met./sec.

It may be incidentally mentioned that a consequent development of the kinetic theory leads to the view that the velocity of dissolved molecules is about as great as that of gaseous molecules, because the osmotic pressure is about as

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