BY ARTIFICIAL MEANS. 245 



operator has only at his disposal the plan of mechanical separation. 



To effect this, a convenient quantity of a diatomaceous culture 

 is introduced into a flask, 7?, previously heated, provided with a 

 sterilised cork, pierced with three holes, in which are fixed three 

 tubes. 



The first tube, Z!, which passes a little way into the neck of 

 the flask, communicating, by means of a tube of caoutchouc, with 

 an earthenware tube, B. The second tube, T'\ goes to within a 

 few centimeters of the bottom of the flask, R \ it carries a stop- 

 cock, r'\ and communicates by a tube of caoutchouc, T'\ with a 

 vessel, v., containing water. This tube forms a siphon intended 

 to draw off the liquid from the flask, R. 



The third tube, T\ has a stopcock, provided with a tuft of 

 filtering wadding ib), and serves to empty and refill the flask. 

 The maceration liquid being introduced into the sterilised flask, 

 and the stopcock, r\ being shut and a vacuum made, the sterilised 

 water comes through the earthenware tube and fills the flask. You 

 suspend the exhaustion, re-establish the pressure of the atmo- 

 sphere, open the stopcock of the siphon (which you prime by a 

 little pressure) ; shut this stopcock and shake the liquid in the 

 flask vigorously. x\fter half-an-hour's rest you decant the liquid by 

 opening the stopcock r, and also r'\ which puts the interior of the 

 apparatus in communication with the atmosphere. 



By this first operation you draw off a great proportion of the 

 bacteria. You do it a second, a third, a fourth, a fifth, and 

 perhaps a sixth time ; finally, the frustules of the diatoms are by 

 this process rendered more numerous than the bacteria, and only 

 till then do you proceed by fracttonne??ient, with the precision 

 needful for bacteriological laboratories. 



In summer, if the temperature is high, the flask should be 

 placed in ice, and the water surrounding the earthen tube should 

 be distilled. Out of from fifteen to twenty macerations, fecun- 

 dated by tht fractionneffient of the liquid from the flask, some will 

 be found that contain no bacteria, which will be shown by the 

 development of tlie diatoms in a maceration or in nutritive 

 gelatine. 



The diatoms, once separated in a state of absolute purity, may 

 be kept indefinitely by employing the well-known precautions 



