COLLECTING BACTERIA FROM THE AIR 43 



cooling, touch one of the colonies and mix lightly with 

 the drop of water without spreading it. Take only a 

 very minute amount of culture, so as to produce the faint- 

 est visible cloudiness in the water. 



4. Smear vaselin around the depression in a hollow- 

 ground slide, quickly invert the cover-slip over the 

 depression, and gendy press the margin on the vaselin. 



5. Examine in oil, using very little light. 

 Brownian movement. — Rub a small amount of carmine 



in a mortar with some water and make a hanging-drop 

 preparation. When examining this through the oil- 

 immersion lens, it will be observed that the small particles 

 of carmine have a lively vibrating motion. This is called 

 "Brownian" or "molecular movement," or "pedesis." 

 The particles scarcely change their relative position. 

 Actively motile organisms, on the contrary, widely change 

 their relative positions. The movement of these may be 

 slow, snake-like, or like a fish swimming; or they may 

 dart rapidly across the field. 



Now observe and describe what changes have taken 

 place in the tube of broth with the hair. Compare with 

 a sterile tube, noting the turbidity, sediment, odor, etc. 

 Also examine in hanging-drop. 



Examine the tube of glucose-agar containing the nail- 

 scrapings. Describe the general appearance, and note 

 whether gas-bubbles are present. 



Finally, from the stained preparations make sketches 

 of the appearance of the culture medium, the shape of 

 the colonies, and the morphology of the organisms. 



Make stained preparations of three different colonies, 

 using the three stains; i. e., gentian-violet, methylene- 

 blue, and carbol-fuchsin. 



