_Spontaneous Generation of Crystals. Optimum Temperature for the Appearance of Germs._—If we consider various supersaturated solutions or liquids in superfusion, we shall soon discover that they can be arranged in two categories. Some remain indefinitely liquid under given conditions unless a crystalline germ is introduced into them. Others solidify spontaneously without artificial intervention, and such crystallization may even be propagated very rapidly under determinate conditions. This implies that these are conditions favouring the spontaneous appearance of germs. This distinction between substances of crystalline generation by filiation and substances of spontaneous crystalline generation is not specific. The same substance may present the two methods of generation according to the conditions in which it is placed. Betol furnishes a good example of this. Liquefy it at 100° in a sealed tube and keep it by means of a stove above 30°, and it will remain liquid almost indefinitely. On the other hand, lower its temperature and leave it for one or two minutes at 10°, and germs will appear in the liquor; prolong the exposure to this degree of heat and the number of these spontaneously appearing germs, appearing in isolation, will rapidly increase. On the other hand, you will observe that propagation by filiation—that is to say, by extension from one to another—is almost absent. The temperature of 10° is not favourable to that method of generation; and we have just seen, in fact, that it is at a temperature of about 70° that extension of crystallization from one to another is best accomplished. The temperature of 70° was the optimum for propagation by filiation. Inversely, the temperature of 10° is the optimum for spontaneous generation. Above and below this optimum the action is slower. We may count the centres of crystallization, which slowly extend further and further, as in a microbic culture one counts the colonies corresponding to the germs primitively formed. To sum up, if there is an optimum for the formation of crystals, there is a different optimum for their rapid extension. _The Metastable and Labile Zones._—This phenomena is general. There is for each substance a set of conditions (temperature, degree of concentration, volume of the solution) in which the crystalline individuals can be produced only by germs or by filiation. This is what occurs for betol above the temperature of 30°. The body is then in what Ostwald has called a _metastable_ zone. There is, however, for the same body another set of circumstances more or less complete, in which its gems appear simultaneously. This is what happens for betol at about the temperature of 10°. These circumstances are those of the _labile zone_ or zone of spontaneous generation.