198 MESSRS. A. V. HARCOUET AND W. ESSON ON THE LAWS OF CONNEXION 



It will be seen that in both cases the quantity of permanganate reduced in three 

 minutes increases with the proportion of oxalic acid up to a certain point ; it then 

 diminishes again until another point is reached ; after which further additions of oxalic 

 acid produce once more a very gradual acceleration. The first part of this result is 

 readily intelligible. The rate at which the reduction proceeds is greater with a larger 

 amount of the reducing agent. But why does a still larger amoimt produce an oppo- 

 site effect 1 The explanation is to be found no doubt in another circumstance by which 

 this reaction is complicated. When solutions of manganous sulphate and potassic 

 permanganate are mixed, a precipitate of hydrated manganic binoxide is produced, 

 which soon separates from the liquid, leaving it colourless if the manganous salt were 

 in excess*. The presence of a sufiicient quantity of oxalic acid prevents this precipi- 

 tation ; a clear brown liquid is formed whose colour very slowly fades, the reduction in 

 a cold and dilute solution occupying many hours. This solution, like that of cupric 

 oxide in potash containing a reducing substance, depends no doubt upon a chemical 

 combination which precedes the mutual action of oxidation and reduction. Probably 

 the brown liquid contains oxalate of manganic binoxide. In the experiments with 

 oxalic acid only, the maximum action in three minutes occurs very near that point at 

 which the amount of oxaUc acid present is five molecules, or the quantity which the 

 measure of permanganic acid can oxidize. If, as the experiments with manganous 

 sulphate appear to prove, the first stage of the reaction consists in the formation of 

 manganic binoxide, this maximum action occurs with one molecule of binoxide and one 

 of oxalic acid. The subsequent minimum again nearly coincides with that point at 

 which ten molecules of oxalic acid have been added for one of permanganate, or two 

 for one of binoxide. It was also observed that whenever a less amount than this was 

 taken, the liquid became turbid, but with this or any larger amount it remained clear. 

 According to analogy, oxalate of manganic binoxide should have the formula MnOg, 

 2C2 O3, or Mn C4 Og. For its formation two molecules of oxalic acid are required. The 

 final action, when not less than two molecules of oxalic acid have been taken, may 

 therefore be thus represented : 



(1) Mn02+2H2C2 04=:2H2 0+MnC4 08. 



(2) MnC^Og =MnC2 04-f-2C02. 



The decomposition of this salt takes place more slowly than does the oxidation of oxalic 

 acid by free binoxide of manganese ; it is accelerated by the presence of sulphuric acid, 

 or of a great excess of oxalic acid. 



In the second series of experiments, in which sulphuric acid was introduced, the 

 amount of action is at first proportional to the amount of oxalic acid taken, for within 

 the allotted three minutes the whole is oxidized. Afterwards the total reduction 

 effected within this time is still increased by the addition of a fourth and fifth and 



* In this case the precipitate consists of 5 Mn 0, , Mn 0. — Goegeu, ' Ann. de Chim. et de Phys.' 3rd series, 

 vol. Ixvi. p. 153. 



