240 THE CULTIVATION OF DIATOMS 



When the microscope shows that the species is in a state of 

 purity — that is to say, without admixture with any other diatoms — 

 you may proceed in order to ensure success to a new dilution less 

 rigorous and only with half-a-dozen macerations. This second 

 operation is intended to convert into certainty the probability 

 already given by the microscopic examination. For even when in 

 a microscopic preparation you can only find diatoms of one kind, 

 it is possible that the macerations may contain others, which, at 

 first very rare, ultimately multiply and become frequent. There- 

 fore, I consider it always indispensable to take a second dilution 

 and a second division with the cultures that are supposed pure. 



I ought to add that these very simple and very practical mani- 

 pulations, that are apt at first to discourage diatomists who are 

 little accustomed to laboratory experiment in bacteriology, offer 

 many advantages, especially as in seeking to separate one species 

 you insulate at the same time many others, which you may culti- 

 vate at leisure if their study does not interest you at the present 

 moment. Thus, in renewing every six months the pure cultures in 

 new macerations, the diatoms will perpetuate themselves indefi- 

 nitely in a state of purity, and the fractional separation need not 

 be renewed for the same species during the life of the observer. 



I give below three examples of the separation of diatoms with 

 the results obtained, each experiment being taken with twelve 

 sterilised macerations : — 



ist exper. 2nd exper. jrd exper. 



