194 MESSES. A. V. HARCOUET AND W. ESSON ON THE LAWS OF CONNEXION 



be possible both to start and terminate the reaction abruptly at a given moment. In 

 the next, either some product or some residue of the action must be a substance for whose 

 estimation exact and ready methods are known, that so the amount of change may be 

 quantitatively determined. Lastly, all the conditions of the reaction must be measurable, 

 or at least definable, and some of them susceptible of modification at will, that thus the 

 influence of each may be examined. 



The first reaction chosen by us for investigation was that of permanganic acid upon 

 oxalic acid. It is well knovra that when a solution of potassic permanganate is added 

 to a solution containing oxalic acid and sulphuric acid, its red colour gradually disap- 

 pears, owing to the reduction of the permanganic acid. The final result of the change is 

 thus represented : 



K2Mn2O8+3H2SO4+5H2C2O4=K2SO4+2MnSO4 + 10CO2 + 8H2O. 



This reaction occurs at a temperature which is easily kept constant and can be exactly 

 measured. It occupies, under duly arranged conditions, a convenient interval of time. 

 The reagents and the products of the reaction are all soluble, and thus the system can 

 quickly be made and wUl remain homogeneous. The reagents are readily obtained in a 

 state of purity, and divided and measured as liquids. The reaction can be stopped 

 abruptly, and the residual permanganic acid estimated, by adding to the solution potassic 

 iodide and determining volumetrically the amount of iodine liberated. Lastly, no varia- 

 tion of light, such as occurred in the course of experimenting*, or of atmospheric pressure, 

 or of any other condition besides those that have been named, affects the result. But 

 the action is not chemically simple. The various complications which it exhibits wiU 

 be discussed as they are revealed by successive experiments. One of these occasions the 

 well-known change of colour from red to brown, which the liquid undergoes as the 

 action proceeds. The manganous sulphate which is formed by the reduction of the higher 

 oxide reacts upon the remaining permanganic acid, producing one or more intermediate 

 oxides which combine with oxalic acid. And since the formation and reaction of man- 

 ganous sulphate could not be avoided, it became necessary to include this salt among the 

 reagents, the effect of whose variation was to be determined. 



A. Variation of Sulphuric Acid. 



The following experiments were made in order to discover the variation in the amount 

 of chemical change due to a variation in the amount of sulphuric acid, the other condi- 

 tions of the reaction being kept constant. In every experiment the solution contained 

 K2Mn2 08, i. e., 316 "2 parts of permanganate of potassium, 5H2 Cg O4, and 2MnS04. 

 The absolute weight of potassic permanganate used in each experiment was "014 gramme, 

 the volume of the solution was 330 cub. centims., its temperature 16° C, and the time 

 allowed to elapse between the mixture of the ingredients and the termination of the 

 action was four minutes. The action was terminated by the sudden addition of an excess 



* Direct sunlight produces a great acceleration. 



