24*2 THE CULTIVATION^ OF DIATOMS 



infusorians that live concurrently with the diatoms. It now 

 remains to point out the method of separating these other living 

 organisms. We shall have to avail ourselves of the fractional 

 method in most cases. 



If the infusoria are very abundant in the artificial cultures, 

 which is not often the case, they are usually got rid of by the 

 operations that enable you to insulate any special diatom. 



If, however, this be not the case, a certain portion of the sedi- 

 ment of the maceration in which the diatoms are living should be 

 introduced into a vessel of distilled water and left for five or six 

 days, shaking it from time to time. In this innutritive liquid the 

 infusoria which live specially on bacteria will quickly disappear, 

 while the diatoms will continue to live, though they will hardly 

 multiply. 



It is essential — especially when you want to study the develop- 

 ment of the diatoms — to get rid entirely of these protozoas, which 

 by their continual movements jostle the diatoms,' change their 

 position, drag them about ; in a word, render all serious and con- 

 tinuous observation impossible. The Rhizopods, besides — as I 

 have had occasion to say — can digest, and consequently destroy, 

 the diatoms, and exercise in the cultures ravages as considerable 

 as those produced by worms and small molluscs. 



It is much less easy to clear out the green algae when they 

 predominate in the sediment or in the cultures. To attain this 

 result, it is necessary to render the siliceous algae predominant. 

 That done, you resort to the fractional method. 



For assisting the predominance of the Pheophyces, the 

 observer has many means at his disposal — means both chemical 

 and physical, 



I, — He will eliminate as far as possible from his macerations 

 the salts of ammonia and potassa. 



2. — He will carry on his cultures under a subdued light in a 

 place where the luminous radiations are barely sufficient to admit 

 of the diatoms increasing, and he will also employ artificial light 

 and modify it by ground or yellow glass. 



3. — He will add to the maceration a dialysed solution of pure 

 silica in the proportion of from i/ioth to i/5th of a gramme of 

 calcined silica to a litre. The green algae, injuriously affected by 



