814 PRINCIPLES OF CHEM1STBY 



is a substance capable of absorbing oxygen for instance, capable o 

 passing into a higher grade of oxidation then the reduction of the 

 permanganic acid into manganous oxides sometimes proceeds directly 

 at the ordinary temperature. This reduction is very clearly seen, 

 because the solutions of potassium permanganate are red whilst the 

 manganous salts are almost colourless. Thus, for instance, nitrous acid 

 and its salts are converted into nitric acid and decolorise the acid solution, 

 of the permanganate. Sulphurous anhydride and its salts immediately 

 decolorise potassium permanganate, forming sulphuric acid. Ferrous 

 salts, and in general salts of lower grades of oxidation capable of being 

 oxidised in solution, act in exactly the same manner. Sulphuretted 

 hydrogen is also oxidised to sulphuric acid ; even mercury is oxidised 

 at the expense of permanganic acid, and decolorises its solution, being 

 converted into mercuric oxide. Moreover, the end point of these reactions 

 may easily be seen, and therefore, having first determined the amount 

 of active oxygen in one volume of a solution of potassium permanganate, 

 and knowing how many volumes are required to effect a given oxidation, 

 it is easy to determine the amount of an oxidisable substance in a 

 solution from the amount of permanganate expended (Marguerite's 

 method). 



The oxidising action of KMnO 4 , like all other chemical reactions, 

 is not accomplished instantaneously, but only gradually. And, as the 

 course of the reaction is here easily followed by determining the amount 

 of salt unchanged in a sample taken at a given moment, 25 the oxidising 

 reaction of potassium permanganate, in an acid liquid, was employed by 

 Harcourt and Esson (1865) as one of the first cases for the investigation 

 of the laws of the rate of chemical change^ as a subject of great import- 

 ance in chemical mechanics. In their experiments they took oxalic acid,- 



parts of sulphuric acid at 100 gives brown crystals of the salt Mn. 2 (SO 4 ) 3 ,H 2 SO 4 ,4H_,0, 

 which gives a precipitate of hydrated -manganese dioxide, H 2 MnO 5 = MnOjH^O, when 

 treated with water. 



Spring, by precipitating potassium permanganate with sodium sulphite and washing 

 the precipitate by decantation, obtained a soluble colloidal manganese oxide, whose 

 composition was the mean between Mn 3 O 5 and MnO 2 namely, Mn a O3,4(Mn0 2 H Q O). 



25 For rapid and accurate determinations of this kind, advantage is taken of those 

 methods of chemical analysis which are known as ' titrations ' (volumetric analysis), and 

 consist in measuring the volume of solutions of known strength required for the complete 

 conversion of a given substance. Details respecting the theory and practice of titration, 

 in which potassium permanganate is very frequently employed, must be looked for in 

 works on analytical chemistry. 



20 The measurements of velocity and acceleration serve for determining the measure 

 of forces in mechanics, but in that case the velocities are magnitudes of length or paths 

 passed over in a unit of .time. The velocity of chemical change embodies a conception of 

 quite another kind. In the first place, the velocities of reactions are magnitudes of the 

 masses which have entered into chemical transformations ; in the second place, these 

 velocities can onlv be relative Quantities. Hence the conceotion of ' velocity ' has auite a 



