132 MESSES. A. Y. HABCOUBT AND W. ESSON ON THE LAWS OF CONNEXION 
between p and t is such that the amount of change within a unit of volume in a unit of 
time is up ; hence X—uptv—ptv . <p(x), and therefore u=<p(x). If we now make a series 
of such sets of experiments, varying x only, and determining the value of u for each set, 
we shall obtain a series of values of <p(x) corresponding to the several values of x , and 
may thence discover what function a, and therefore X, is of x. And if we are able thus 
to determine what function % is of each of the conditions a, 1), c, . . ., we shall obtain an 
equation expressing the laws of connexion between all the conditions of this chemical 
change and its amount. It is, however, to be observed, that for the determination of u 
in each set of experiments it is necessary that the introduction and variation of the con- 
dition x should not alter the law of connexion between p and t. In other words, to 
determine <p(x) we need to obtain a series of equations of the following form, 
P P—tfM-f) . . . . 
p 7 p 7 p 
It is plain that the investigation of a single reaction thus considered is a work requiring 
the performance of a very large number of experiments. In the case before us it is 
possible to react with the hydric peroxide either upon hydric iodide by itself, or potassic 
iodide and hydric sulphate, or to substitute sodic iodide for potassic, or hydric chloride 
for hydric sulphate, without (as appears from the numbers in Table IV.) altering the 
nature of the reaction. And there is little doubt that the system might be modified by 
the introduction or substitution of many other substances, without affecting the chemical 
change, except as regards its rate. Each of the conditions thus introduced furnishes 
a fresh subject of inquiry, namely, as to the effect of a variation in its amount upon that 
of the chemical change. On the effect of varying some of these conditions of the reac- 
tion we have made numerous experiments, especially on the variation of the amounts of 
potassic, sodic, and hydric iodide, hydric sulphate, hydric chloride (substituted for hydric 
sulphate), and of the temperature of the solution. We propose to include in the pre- 
sent communication an account of the results we have obtained by varying in successive 
sets of experiments the amount of iodide. 
Variation of Iodide. 
When two salts are mixed in solution and no precipitation occurs, or 'change in the 
colour of the liquid, it is not possible to arrive at any probable conclusion as to the 
proportions or nature of the salts which the solution contains. In the greater part of 
the experiments about to be recorded, potassic, or sodic, or hydric iodide was added to 
a liquid containing an excess of hydric sulphate or chloride. Whether the systems thus 
formed contained the salts introduced into them, or hydric iodide and a metallic sulphate 
or chloride, or whether the metallic iodides were partially decomposed by the excess of 
free acid, we are unable to say. It may be hoped that the further investigation of the 
effect of varying these substances will throw light upon this question ; but at present we 
must consider our results generally as depending upon the variation of iodide. 
The following Table contains the numerical results of five sets of experiments, in each 
