SPONTANEOUS GENERATION. 297 



Let US return to generation, properly so called, and 

 find in it the characteristics of brute bodies and of 

 crystals. 



TJie Solving of Micro-orgamsms. — When a micro- 

 biologist wishes to propagate a species of micro-, 

 organisms, he places in a culture medium a few 

 individuals (one is all that is actually necessary), and 

 soon observes their rapid multiplication. Usually, if 

 only the ordinary microbes in atmospheric dust are 

 wanted, the operator need not trouble to charge the 

 culture; if the culture tube remains open and the 

 medium is suitably chosen, some germ of a common 

 species will fall in and the liquid will become 

 colonized. This has the appearance of spontaneous 

 generation. 



TJie Sowing of Crystals. — Concentrated solutions of 

 various substances, supersaturated solutions of sodium 

 magnesium sulphate, and sodium chlorate are also 

 wonderful culture media for certain mineral organic 

 units — certain crystalline germs. Ch. Dufour, ex- 

 perimenting with water cooled below 0° C, its point 

 of solidification ; Ostwald, with salol kept below SQ^-S, 

 its point of fusion ; Tammann, with betol, which 

 melts at 96° ; and, before them, Gernez, with melted 

 phosphorus and sulphur — all these physicists have 

 shown that liquids in superfusion are also media 

 specially appropriate for the culture and propagation 

 of certain kinds of crystalline individuals. 



Some of these facts have become classic. Lowitz 

 showed in 1785 that a solution of sodium sulphate 

 could be concentrated by evaporation so as to contain 

 more salt than was conformable with the temperature, 

 without, however, depositing the excess. But if a 

 solid fragment, a crystal of salt, is thrown into the 



