THE LIFE OK MATTER. 427 



sealed at a temperature (vf lOU . it may be cooled to lower and lower 

 temperatures, to+7<» , to^-25'-','{o+l'* , to — 5 , without solidifying'. 

 Let us suppose that by some combination of circumstances a small num- 

 ber of centers of crystallization — that is to say, of crysttdline germs — 

 has appeared in the solution. Solidification will extend slowly at the 

 ordinary temperaturi's of 20'- to 25" and tho neighboi'hood. On the 

 other hand, it will ])e propagated with great rapidity if the liquor is 

 kept at about 70 . This point, 70 , is the thermic o})tinunu for the 

 propagation of germs. It is the most favora))le temperature for what 

 might be called their incubation. As soon as tiie germs tind them- 

 selves in a licjuor at 7o they increase, nudtiply. and sho\\ that they 

 are in the best conditions for growth. 



Span tcnu Otis generdthni of cry dais — Optimuui toirpiivtiire for the 

 apjjeai'antr <f <jt-riiis. — If we consider various supersaturated solu- 

 tions or liquids in surfusion, 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 pr-opagated very rapidly under determinate 

 conditions. These are for them conditions favoring the spontaneous 

 appearance of germs. 



This distinction ])etween substances possessing crystalline gem^ra- 

 tion l)y tiliation and substances having crystalline generation that 

 arises spontaneously 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 

 loo in a sealed tube and keep it by means of a stove abo\'(> 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 will rapidly 

 increase. On the other hand, 3"Ou will observe that propagation by 

 filiation, that is to say, by extension from one to another, is almost 

 absent. The temperature of IC-' is not fa\'orable 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 s})ontaneous generation. Above and below this optimum 

 the action is slower. We may count the centers of crj^stallization 

 which slowl}' extend farther and farther, as in a microbic culture one 

 counts the colonies corri'sponding to the germs primitively formed. 

 To recapitulate, if there is an optinuun for the formation of crystals 

 there is also one^ difi'erent fi'om the tirst, for th«Mr rapid extension. 



