chemical change by measurement of the gases evolved. 603 
In Fig. 9 is given the result of plotting, according to the 
method explained above, all the observations in the two experi- 
ments from which the above sets of numbers were taken. The 
curve OP A represents the evolution of gas in the experiment in 
which the reaction mixture was agitated to remove the gas which 
is supersaturated in the solution ; PB that in which the solution 
was at rest. It will be noticed that a point of inflexion occurs in 
the curve PB at about the point B, the rate of evolution of gas 
being greatest at this point. This corresponds to the maximum 
vertical distance AB between the two curves, this distance repre- 
senting the amount of gas which remains supersaturated in the 
solution at rest. By drawing tangents to the curves the rates of 
evolution of gas at different points may be determined, and curves 
may be drawn in which the rates of evolution thus found are 
plotted in terms of the amount of gas which has been given off. 
Time, minutes 
Fig. 9. 
Fig. 10 gives the results obtained in this manner. It will be seen 
that the points which constitute the curve ODE lie very closely 
on a straight line, thus showing as before that the reaction as 
measured by the rate of evolution of gas from the agitated solution 
is unimolecular. On the other hand the curve GHE very clearly 
shows that the rate of evolution of gas from the unstirred solution 
rapidly increases from zero till the maximum value is reached at a 
point corresponding to an evolution of about 12 c.c. of gas. From 
this point the rate begins to diminish and continues to fall till all 
the gas is evolved. 
It will be seen that between the points H and F the curve 
approximates to a straight line and for a short range the reaction 
appears to be nearly unimolecular. Near the end of the reaction 
however when a chemical change is complete, as shown by the rate 
of evolution of gas from the stirred solution, the gas given off 
from the unstirred solution is simply that which remains stored up 
