158. Besides the case now mentioned, there are other instances in which, no doubt, molecular separation becomes gradually changed into heat motion. Thus, when a piece of glass has been suddenly cooled, its particles have not had time to acquire their proper position, and the consequence is that the whole structure is thrown into a state of constraint. In the course of time such bodies tend to assume a more stable state, and their particles gradually come closer together. It is owing to this cause that the bulb of a thermometer recently blown gradually contracts, and it is no doubt owing to the same cause that a Prince Rupert’s drop, formed by dropping melted glass into water, when broken, falls into powder with a kind of explosion. It seems probable that in all such cases these changes are attended with heat, and that they denote the conversion of the energy of molecular separation into that of molecular motion. 159. Having thus examined the transmutations of (C) into (D), and of (D) back again into (C), let us now proceed with our list, and see under what circumstances absorbed heat is changed into _chemical separation_. It is well known that when certain bodies are heated, they are decomposed; for instance, if limestone or carbonate of lime be heated, it is decomposed, the carbonic acid being given out in the shape of gas, while quick-lime remains behind. Now, heat is consumed in this process, that is to say, a certain amount of heat energy absolutely passes out of existence _as heat_ and is changed into the energy of chemical separation. Again, if the lime so obtained be exposed, under certain circumstances, to an atmosphere of carbonic acid, it will gradually become changed into carbonate of lime; and in this change (which is a gradual one) we may feel assured that the energy of chemical separation is once more converted into the energy of heat, although we may not perceive any increment of temperature, on account of the slow nature of the process. At very high temperatures it is possible that most compounds are decomposed, and the temperature at which this takes place, for any compound, has been termed its _temperature of disassociation_. 160. Heat energy is changed into _electrical separation_ when tourmalines and certain other crystals are heated.