BY ARTIFICIAL MEANS. 243 



this substance, develops very slowly in silicified maceration, whilst 

 the diatoms love silica and do not hesitate to get the better of the 

 Chlorophyces. In default of a solution of silica, the experi- 

 menter may do a great deal by adding to his macerations a little 

 finely carded asbestos. 



When a partial deposit or yellow patches of Diatoms are per- 

 ceived in cultures conducted after this manner, a small quantity is 

 taken in a pipette, to start a new maceration. It is rare that 

 among these sowings the green algae are able to maintain their 

 position above the diatoms. When the predominance of these 

 latter is secured, you separate them by the method of division 

 previously described. 



The elimination of Fungi must be secured as well as that of 

 the green algae. The mixture of Fungi and Diatoms is thrown 

 into sterilised distilled water ; that is to say, water that has been 

 cleared of all hydro-carboniferous matters. The greater part of the 

 fungi will die in this medium, whilst the diatoms and the green 

 fungi will acquire a certain development, especially if to the dis- 

 tilled water — which is known to be free from all organic matters — 

 a few drops per litre of the solutions A and B be added. "^ The 

 formula of these solutions was given in the first chapter of this 

 essay. 



It now remains to isolate the Diatoms from the Bacteria. This 

 is one of the most difficult manipulations in the technique of the 

 diatomist, for you can only get rid of the bacteria by a series of 

 delicate operations conducted in a most careful manner and 

 requiring special training. 



From the moment when the experimenter has determined to 

 clear his macerations of bacteria, he must no longer be content to 

 sterilise his mixtures at 70^ C. All, without exception, must be 

 heated to iio^ C. for at least half-an-hour, or sterilised by filtra- 

 tion through earthenware ; and in conclusion the glass vessel 

 should be raised gradually to 180° C. As the macerations cannot 

 support without injury to their nutritive power a temperature of 

 iio'^ C, they should be prepared and sterilised by cold and then, 

 carefully protected from the action of the atmosphere, should be 

 placed in the vessels that have been carefully raised to i8o°C. 



* See page 37. 



