300 LIFE AND DEATH. 



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 + IO, to 5 without solidifying. 

 Let us suppose that by some combination of circum- 

 stances 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 pro- 

 pagation 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 in- 

 definitely 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 



