BETWEEN THE CONDITIONS OF A CHEMICAL CHANGE AND ITS AMOUNT. 135 
The mean values of a derived from these sets of experiments are compared in the 
following Table with a series calculated from the equation et= ’01347%. 
Table VIII. 
- 
Observed. 
a. 
Calculated. 
1 
•0136 
•0135 
2 
•0268 
•0269 
3 
•0404 
•0404 
4 
•0538 
•0539 
5 
•0672 
•0673 
6 
•0804 
•0808 
7 
•0948 
•0943 
8 
•1080 
•1078 
Thus it appears that the amount of chemical change occurring in the solutions at any 
moment varies directly with the amount of iodide, if all the other conditions are the 
same. A few of the numbers from which the mean values of a are obtained differ con- 
siderably one from another. These differences were generally due to observed errors in 
the management of the temperature of the solution, which having through inadvertence 
risen or fallen a little during one interval, was made to fall or rise in a corresponding- 
degree during the next interval, that the mean result might be correct. 
In both these series the quantity of iodide was small in proportion to the quantity of 
acid, amounting at the most to 4 per cent. Two sets of experiments were subsequently 
made with systems containing in a cubic centimetre 54’5 ff 2 S 0 4 , and 10’42, 20’84 El 
respectively, at a temperature of 17° C. The values of a given by these two sets were. 
•0116 and -0243, the latter of which is considerably more than double the former. In 
some other sets of experiments, in which instead of a metallic iodide different quantities 
of hydric iodide were added to the solutions, it was observed similarly that the increase 
in the rate of change was more than proportional to the increase of hydric iodide. Now, 
since it appears that hydric sulphate or hydric chloride, though playing no immediate 
part in the reaction, yet accelerates its course, it seems reasonable to suppose that a 
double effect may be produced by the addition of hydric iodide. For while on the one 
hand this addition increases the amount of substance which the hydric peroxide has to 
act upon, on the other hand, like the addition of hydric sulphate or hydric chloride, it 
increases the acidity of the solution. But further, the rate of change depends not only 
upon the acidity of the solution, but upon the particular acid which it contains. The 
two acids upon which we have experimented affect the rate of change in different degrees, 
hydric chloride exercising a greater influence than hydric sulphate ; so that if we were 
to add a quantity of neutral chloride to a solution containing hydric sulphate, we should 
doubtless increase the rate of change ; for some of the hydric sulphate would be replaced 
by its equivalent (in the ordinary chemical sense) of hydric chloride. In the same way 
it is probable that a solution into which hydric sulphate and potassic iodide have been 
introduced contains some hydric iodide, and that this acid also has its particular effect 
t 2 
