is greater than the thermal effect observed (44 C.) by the sum of that quantity of heat required for the decomposition of one molecule of chlorine and that necessary for the de- composition of one molecule of hydrogen.
Besides, we know that this reaction, although capable of evolving much heat, does not take place when the two elementary gases are mixed. It requires to be started by the action of light or of heat, that is, by the co-operation of external energy, because it is necessary to weaken to a certain extent the force which unites the similar atoms, so that the preponderating attraction between the hetero- geneous atoms may overcome this and bring about the change. Combination started at a point quickly propagates itself through the whole mixture, on account of the heat evolved by the reacting particles. Therefore, in the case of direct combination the quantity of heat evolved must be subjected to a correction in order to give a measure of the chemical affinity. We do not know how to make this cor- rection, as we do not know how much heat is dissipated in separating the similar atoms.
The thermo-chemical numbers, therefore, give us only a rough measure of affinities, with an approximation which we cannot estimate. Nevertheless, on reviewing a table of Heats of Formation we at once notice that the most stable compounds, those which we are in the habit of regarding as containing the strongest affinities, are those which have the greatest heat of formation.
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