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 is melted in tubes sealed at a temperature of 100° C., it may be cooled to lower and lower temperatures—to + 70°, to + 25°, to + 10°, to - 5° without solidifying. Let us suppose that by some combination of circumstances a few centres of crystallization—that is to say, of crystalline germs—have appeared in the solution. Solidification will extend slowly at the ordinary temperature, at 20° to 25° and thereabouts. On the other hand, it will be propagated with great rapidity if the liquor is kept at about 70°. This point—70°—is the thermal optimum for the propagation of germs. It is the most favourable temperature for what may be called their incubation. As soon as the germs find themselves in a liquor at 70° they increase, multiply, and show that they are in the best conditions for growth.

''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