350 On the Time required by Compounds for Decomposition. 
could be predicted if the heat of combination of the elements 
were known; for, ceteris paribus, the bodies which produce most 
heat in chemically uniting always combine to the exclusion of 
others. This is, however, only a repetition of the truth proved 
by the experiments of Mr. Joule and myself, that the heat of 
combination is in proportion to the affinity of the combining 
bodies. If, therefore, I say, experiments were instituted to find 
the amount of heat evolved by the combination of the elements, 
the results would enable us in a great degree to calculate a priori 
chemical phenomena, 
The method which I have described for finding the heat ab- 
sorbed when a fluid is decomposed, viz. by making it the electro- 
lyte in contact with the negative plate of a pair, and taking the 
needle of the galvanometer as the index of the current passing, 
and consequently of its rapidity, and therefore of the heat ab- 
sorbed (as one depends on the other), offers, I think, great faci- 
lity for experimenting on this subject; the galvanometer here 
acts the part of a thermometer of chemical action, the needle 
varying with the amount of heat absorbed by the decomposition, 
just as the mercury does in the common thermometer for changes 
of temperature. I endeavoured last year to find the amount of 
heat produced by the chemical combination of oxygen with 
several elements; an abstract of the experiments is published in 
the Proceedings of the Royal Society for January 1856. The 
results of the experiments show, that when oxygen combines with 
several bodies, the quantity of heat produced is not in each in- © 
stance the same, but is given out in multiple proportion. If the 
heat of combination be known, we can also precalculate with 
precision the quantity and intensity of any proposed galvanic 
arrangement. We have only to find the difference of the 
amount of heat generated at the positive end by the combination, 
and that absorbed at the negative end by decomposition. I 
would just mention, that in this way I prejudged that solution 
of iodic acid used instead of nitric acid, would give the same 
quantity and intensity as the latter, when substituted for it in a 
Groye’s pair; and I found by experiment that the idea was cor- 
rect. The decomposition of iodic acid does not absorb more 
heat than that of nitric acid, and therefore allows the current to 
pass with equal rapidity ; consequently the electromotive force 
is the same. However, as iodine is thrown down, this accumu- 
lates on the platina, and the conducting power of the plate is 
interfered with. Still, as a proof of the value of a knowledge of 
the heat evolved by chemical combinations in precalculating the 
result of galvanic arrangements, the experiment was successful. 
In conclusion, I would beg to remark that the subject of this 
paper, viz. the ¢ime that compounds take to decompose, has not 
