< 200 MESSRS. A. V. HARCOTJET AND W. ESSON ON THE LAWS OF CONNEXION 



The experimental numbers in the two Tables are to be regarded as a series of 

 Ineasurements of the distance of a point on each curve from the axis of x, along an ordi- 

 nate whose distance from the axis of y represents an interval of three minutes. 



Sulphuric acid appears to accelerate the reaction in two ways. First, it promotes the 

 action of manganic binoxide on oxalic acid ; and secondly, it retards the formation of 

 the comparatively stable compound, which in its absence is at once produced on mixing 

 the three other ingredients. This is shown by the persistency of the red colour of per- 

 manganic acid in a mixture containing a large proportion of sulphuric acid. To the 

 former cause principally must be attributed the greater rate of change throughout the 

 second series, and to the latter the circumstance that in this case the maximum action 

 occurs with rather more than five molecules, and the subsequent minimum with more 

 than ten. It seems probable that with a large excess of sulphuric acid the rate at 

 which the reduction proceeds would be found to vary directly with the quantity of 

 oxalic acid, according to the law which appears to exist in other similar cases. 



D. Variation of Time. 



One of the conditions which has been kept constant hitherto, is the time during 

 which the reaction has been allowed to proceed. But in order to discover the exact 

 effect of each chemical or physical variation, it is necessary to be acquainted with the 

 whole course of the reaction, and not merely with the amount of change accomplished 

 under each set of circumstances during one interval of time. Now, by performing a 

 number of experiments only differing one from another in the period which is allowed 

 to elapse between starting and stopping the reaction, we may trace its course with any 

 required degree of minuteness. And if we can discover the relation of the series of 

 numbers representing the duration of the several experiments to those representing the 

 corresponding amounts of chemical change, an expression may be thence deduced for the 

 rate at which the reaction is proceeding at a given time, or with a given quantity of 

 substance, and a comparison made between the rates derived from different series of 

 experiments. 



If it were possible for all the conditions of a chemical change to remain constant, if, 

 for example, the substances reacting could be added in proportion as they disappeared, 

 and those formed either were without influence or could be removed, the effect of a 

 variation of time might be confidently predicted. In such a case the total amount of 

 chemical change would be directly proportional to the duration of the action. But 

 where one or more of the substances diminishes in quantity as the change proceeds, the 

 relation is no longer of this simple character. 



The following series of experiments, in which only the duration of the reaction was 

 varied, had for its object the discovery of this relation. The various substances were 

 employed in the proportions in which they react vrith one another, except the sulphuric 

 acid, of which a double portion was taken. After the other solutions had been mixed 

 and brought to the right volume and temperature, the measure of potassic perman- 



