From the short description which has been given, it is seen that the oxides of iodine and their compounds are, as a rule, more stable than those of bromine and chlorine, and this is connected with the heat which is evolved or absorbed during their formation. This heat is seldom determined directly; never when the compounds are produced with absorption of heat. Thus, when chlorine combines with oxygen to form C1₂O, enough heat is absorbed to cool 17,800 grams of water through i°, or what is the same thing, on decomposing C1₂O heat enough is liberated to raise the temperature of 17,800 grams of water through i°. This is termed the heat of formation of the substance. The heat of formation of chloric acid from chlorine, oxygen, and water involves a heat-absorption of 20,400 calories, and these substances are both very unstable. On the other hand, the combination of iodine with oxygen is attended with an evolution of heat of 25,300 calories, and an additional 2600 calories are liberated when it combines with water to form iodic acid. Perchloric acid, too, is formed with evolution of heat (4200 calories), and thus iodic, periodic, and perchloric acid are comparatively stable. The heat-change during the formation of a compound, therefore, is connected with its stability, although the exact relationship between the two is at present unknown.