194 
DE. A. VERNON HARCOURT ON THE VARIATION WITH 
change is m times the former change. The number m may be regarded as the 
measure of the effect of heat energy upon chemical energy. 
In the Bakerian Lecture it was shown that m is constant for all tlie temperatures 
at which the experiments were made, hut varies with the medium in which the 
chemical change takes place. The constancy of m for each medium is secured b}^ 
making one of the constituents of the medium predominant; m has different values 
for different predominant constituents in the medium in which the same chemical 
change occurs; m has also different values for different chemical changes. On p. 879 
of the Lecture the values of m are 10, 20‘4, 21‘2, 24T for the predominant 
constituents NaHCO'^, HCl, HI respectively in the medium in which the 
reaction between hydrogen peroxide and hydrogen iodide is taking place. The value 
of m is 40'5 in the case of the reaction between hydrogen chlorate and potassium 
iodide, studied by Pendlebury and Seward, and quoted on the same page of the 
Lecture. 
If the conditions for a stable communication of heat energy to chemical energy are 
not satisfied the value of m is different at different temperatures. For instance, the 
predominant constituent of the medium may change its character with a change of 
temperature, or the nature of the reaction between the substances undergoing change 
may vary. Examples of this variation of m with temperature are given later. But 
even when m varies with temperature it is still to be regarded as the measure of the 
effect of heat energy upon chemical energy at a given temperature, and the value of 
m is obtained from the experiments by calculating the ratio of dklk to dT/T at each 
temperature. In the experiments recorded in the preceding pages there are no such 
disturbing conditions, and m is constant at all the temperatures at which these 
experiments were made. 
In the experiments recorded in the preceding pages the expression for the progress 
of the chemical change is, by the law of mass action, of the form 
f {y)di/ldx = -k, 
y being the concentration of one of the substances undergoing change, and x the 
time during which the change takes place. This expression leads to the relation 
/(d2)-/(di) = 
The time x is the observed time during which the potential chemical energ}^ 
expressed byy’(^i) had changed to the potential chemical energy expressed by f {y^. 
In the above experiments this change of energy is indicated by the passage of the 
solution from one given colour to another. At different temperatures this change of 
energy is constant. Thus if k' are the rates of change at temperatures T, T' 
respectively, and x, x' are the corresponding observed times. 
f{y 2 ) -/(^i) = kx ^ k!x'. 
