SPONTANEOUS GENERATION. 3OI 



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 tem- 

 perature of about yo" 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 circum- 



