Heat also creates a kind of _forced equilibrium_. This becomes but slowly modified, so that a body may remain for a long time in a state which is, however, not the most stable for the conditions under which it is considered. The number of these bodies _not in equilibrium_ is as great as that of the substances which have been exposed to fusion. All the Plutonic rocks are in this condition. Glass presents a condition of the same kind. Thermometers placed in melting ice do not always mark the zero Centigrade. This displacement of the zero point falsifies all records if care is not taken to correct it. The correction usually requires prolonged observation. The theory of the displacement of the thermometric zero is not entirely established; but we may suppose, with the author of the _Traité de Thermométrie_, that in glass, as in alloys, are to be found compounds which vary according to the temperature. At each temperature glass tends to assume a determinate composition and a corresponding state of equilibrium; but the previous temperature to which it has been subjected clearly has an influence on the rapidity with which it attains its state of repose. The effect of variation is more marked when we observe glass of more complicated composition. We can understand that those which contain comparable quantities of the two alkalies, soda and potash, may be more subject to these modifications than those having a more simple composition based on a single alkali. _Effects of Annealing._—A piece of brass wire that has been drawn and then heated is the scene of certain very remarkable internal changes, and these have been only recently recognized. The violent treatment of the metallic thread in forcing it through the hole in the die has crushed the crystalline particles; the interior state of the wire is that of broken crystals embedded in a granular mass. Heating changes all that. The crystals separate, repair themselves, and are built up again; they are then hard, geometrical bodies, in an amorphous, relatively soft and plastic mass; their number keeps on increasing; equilibrium is not established until the entire mass is crystallized. We may imagine how many displacements, enormous when compared with their dimensions, the molecules have to undergo when passing through the resisting mass, and arranging themselves in definite places in the crystalline structures. In the same way, too, in the manufacture of steel, the particles of coal at first applied to the surface pass through the iron. This _faculty of molecular displacement_ enables the metal in some cases to modify its state at one point or another. The use made of this faculty under certain circumstances is very curious, greatly resembling the adaptation of an animal to its environment, or the methods of defence against agents that might destroy it.