Prof. Ostwald on Chemical Action at a Distance. 149 



trical charge can only be directly removed from the ions when 

 they come into contact with electrodes of the same nature. 

 According to the present condition of our knowledge this 

 occurs only in cases of positive ions, which consist of metal 

 atoms, in contact with electrodes of the same metal. A 

 system, for example, of two zinc plates in zinc-sulphate solu- 

 tion allows the weakest galvanic current to pass through and 

 takes up no charge (not considering the secondary actions 

 from alterations in concentration, which do not concern us 

 here) ; while, for example, a system of two platinum plates in 

 dilute sulphuric acid allows a continuous current to pass only 

 upon the application of electromotive forces of definite amount 

 (over 1*8 volt), after a not inconsiderable quantity of elec- 

 tricity has gathered upon the plates in the form of a charge. 

 We must conclude from this that, for the transfer of electricity 

 from an ion to a heterogeneous electrode, a definite difference 

 of potential is in general requisite, which is to be regarded as 

 dependent upon the nature of the ion as well as upon that of 

 the electrode. 



To return to the above-mentioned case, it will depend upon 

 the nature of the positive ion and of the electrode whether 

 the former, under the influence of the difference of potential 

 caused by the presence of the zinc, will give up to the platinum 

 its electricity or not. If the ion be the potassium of potassium 

 sulphate, which holds the electricity very firmly and only gives 

 it up under the action of potential differences of between two 

 and three volts, no transfer of the electricity will take place. 

 If, however, the potassium sulphate at the platinum wire be 

 replaced by sulphuric acid, the difference of potential then at 

 hand is sufficient to force the hydrogen ions thereof to part 

 with their electrical charges, whereupon they escape in the 

 form of ordinary hydrogen. By the combination of this posi- 

 tive electricity with the negative of the metal the latter is 

 partially neutralized, the zinc, in consequence of its lowered 

 negative potential, becomes able to send out more positive 

 ions, and the action continues as long as metallic zinc or 

 hydrogen ions remain at disposal. 



This explanation makes apparent why in this case the 

 acid must be at the platinum and not at the zinc ; it throws 

 light at the same time upon all the peculiarities which are 

 manifested when zinc is dissolved in acids, and which have 

 till now remained insufficiently explained. It forms at the 

 same time the starting-point towards an understanding of the 

 phenomena connected with the dissolving of the metals in 

 general. 



Pure zinc does not dissolve in acids, as was found by De 



