128 



BACTERIOLOGY. 



slide is then pressed firmly down on gold-leaf or tinfoil and allowed 

 to dry. When dry, the metal is scraped away, leaving two triangles 

 with a small interval between them. 



FIG. 59. SLIDE WITH GOLD-LEAF ELECTRODES. 



The liquid containing the micro-organisms is placed between the 

 electrodes, covered with a cover-glass, and then subjected to the 

 electric current. 



(G) METHODS OF EMPLOYING AND STORING LIQUID MEDIA. 



Cultivations in liquid media can be carried on in test-tubes, but 

 it is more satisfactory to employ special forms of flasks, bulbs, and 

 U tubes, such as those employed by Pasteur and his school, and by 

 Lister, Sternberg, and Aitken. 



Lister's flasks. These flasks were especially introduced by Lister 

 as a means of storing liquid nutrient media. 

 They are so constructed that after removal 

 of a portion of the contents, on restoring 

 the vessel to the vertical position, a drop of 

 liquid always remains in the extremity of 

 the nozzle, which prevents regurgitation of 

 unflltered air. 



Sternberg' s Bulbs. The method of intro- 

 ducing liquid into the bulb employed by 

 Sternberg, and of sterilising and inoculating 

 it, is as follows : The bulb is heated slightly 

 over the flame, and the extremity of the neck, after the sealed point 

 has been broken off, is plunged beneath the surface of the liquid. 

 As the air cools the liquid is drawn into the bulb, 

 usually filling it to about one-third of its capacity. ~^ = ^^====== 



The neck of the flask is again sealed up, and the 



liquid which has been introduced is sterilised by IG ' ,'~ TERN- 

 BERG'S BULB. 

 repeatedly boiling the flasks in the water- bath. 



They should then be placed in the incubator for two or three days ; 



FIG. 60. LISTER'S 

 FLASK. 



