BETWEEN THE CONDITIONS OF A CHEMICAL CHANGE AND ITS AMOUNT. 121 
and hyposulphite, which furnishes the method of our investigation. But whereas hydric 
peroxide acts on hydric iodide much more slowly than iodine acts on hyposulphite, hydric 
permanganate appears under the same circumstances to form iodine even more rapidly 
than it is reduced. So that in this case it is possible actually to see the double reaction,, 
for each drop of permanganate as it enters the fluid developes for an instant the colour 
of iodine. But the fact of the alternate action is proved independently of this observa- 
tion ; for while, as has been stated, the result obtained by the addition of permanganate 
to the mixture of hydric iodide and hyposulphite is the same as that obtained when the 
two reactions are caused to occur successively, the result of the direct action of hydric 
permanganate on hydric hyposulphite is widely different. 
The relation between the measure of peroxide and the drops of hyposulphite may also 
be determined in another manner. It is possible at the close of the actual set of expe- 
riments, having allowed the liquid in the cylinder to stand until the action has come 
practically to an end, to determine the excess of hyposulphite by means of a standard 
solution of iodine or permanganate, and then to determine by the same means the value 
of an entire drop subsequently added. Hence we know what fraction of a drop, in 
addition to the entire drops previously introduced, has been required to reduce the whole 
of the peroxide, and this quantity (the whole number and the fraction) expresses the 
value of the peroxide at the commencement of the experiment. If we represent by r 
the amount of residual hyposulphite at the close of the experiment, and by d the amount 
in one drop, and by n the number of drops added between the first and last appearances 
of the blue colour, then ^ is the fraction of a drop which remained unacted upon, and 
is the fraction of a drop acted upon by the last portion of peroxide ; and putting j) 
d—r 
equal to — n-\-p is the whole quantity acted on, and may therefore represent also the 
quantity of peroxide at the moment of the first observation. At the moment of the 
second observation the quantity of peroxide is n — 1 -\-p, and at the moments of subse- 
quent observations it is successively n-2-{-p, n-o-\-p, &c., until finally at the moment 
of the last observation only p remains. Now the decrease of the peroxide is a measure 
of the amount of chemical change. Each time that the operation represented by 
H 2 0 2 +2HI=2H 2 0+I 2 
is performed a molecule of peroxide disappears. We may therefore regard the change 
by which n-\-p parts of peroxide become n— 1+JJ parts as a definite portion of chemical 
change. Representing, then, the observed times by £ 0 , t 2 , &c., t 1 — t Q , t 2 —t l , &c. are 
the successive intervals in which as the experiment proceeds this portion of chemical 
change is accomplished. Now if all the conditions of the reaction could be kept con- 
stant, if it were possible to reconvert the water which is formed into hydric peroxide, as 
it is possible, by placing sodic hyposulphite in the solution, to reconvert the iodine which 
is formed into, hydric iodide, then, the same event occurring always under the same con- 
