BETWEEN THE CONDITIONS OF A CHEMICAL CHANGE AND ITS AMOUNT. 129 
at the commencement of its motion has a certain amount of potential energy capable of 
being transformed into actual energy. As the body falls the potential energy gradually 
becomes actual. Each experiment supplies data for the determination of the following 
quantities : — 
(1) The initial potential change. 
(2) The final potential change. 
(3) The actual change. 
(4) The time during which the actual change has occurred. 
The relation existing between these quantities has been found to be of such a nature 
that the ratio of the initial and final potential changes in a given system depends only 
upon the time of the actual change, so that if this time is constant the ratio is constant ; 
and since the actual change is simply the difference between the initial and final potential 
changes, it follows that for equal intervals of time the actual change is proportional to 
the initial potential change. Now if we could construct a system in which the potential 
change remained constant, it is clear that the actual change would proceed at a uniform 
rate, depending upon the quality of the system and proportional to the constant potential 
change. In all the systems upon which our experiments have been made the potential 
change varies, so that we are not able directly to observe this uniform rate, but we can 
obtain its value indirectly in the following way. 
Suppose the time of actual change to be so small that its rate may be considered 
uniform during that time, the actual change will be so small that the initial and final 
potential changes may be considered to be equal ; in other words, the potential change 
will be constant. The ratio of the small actual change to the time of its occurrence will 
thus represent the uniform rate of actual change when the potential change remains 
constant. The equation which connects the initial and final potential changes y , y' with 
the time of actual change has been found to be 
y’~ e ’ 
whence we obtain 
dy 
~Tt= a y- 
Now — dy is the actual change which occurs during the time dt, and from what is stated 
above the ratio of these small quantities is the uniform rate of actual change when the 
potential change y remains constant. It follows therefore that in a given system, in 
which there exists a constant quantity of potential change y, the uniform rate of actual 
change is ay. Or since a is a constant for the given system, the rate of actual change is 
proportional to the potential change. If the unit of time is one minute, a represents 
the fraction of the potential change which is converted into actual change in one minute. 
We may represent what we have spoken of as potential change by writing separately 
the left-hand side of a chemical equation. For example, the actual change in this 
reaction being represented by H 2 0 2 +2H I=2H 2 0+I 2 , the corresponding potential 
