176 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 



both slides and cover-glasses. The cleaned glassware is removed from 

 the cleansing bath and washed with running water until neutral to 

 litmus paper. It is stored either in slightly ammoniacal alcohol, or 

 dried with a soft cloth, previously freed from grease by boiling in a 

 5 per cent, sodium carbonate solution. 



I. Examination of Living Bacteria. A. Hanging Drop. The motil- 

 ity, shape, and size of bacteria may be studied in a "hanging-drop" 

 preparation. A drop of fluid from a bacterial culture in liquid media 



FIG. 10. Hollow-ground slide for hanging drop. 



is transferred to the center of a thin cover-glass. If the growth is 

 upon solid media a drop of physiological salt solution 1 is placed upon 

 the center of the cover-glass as before, and a very small amount of 

 the culture is removed with a platinum needle and emulsified in it. 

 Next, the rim of the concavity in a "hollow-ground slide" is ringed 

 with vaselin and the cover-glass is inverted over it in such a manner 

 that the drop is suspended in the hollow, but touches neither the 

 sides not the bottom. The vaselin seals the preparation, causing it 

 to adhere to the slide, and also prevents evaporation. The prepara- 

 tion is now ready for microscopic examination. The one-sixth or one- 

 eighth-inch objective should be used, with the diaphragm partly closed 

 to reduce the intensity of illumination. It is desirable to focus first 

 upon the edge of the drop; the edge is sharply defined and readily 

 located. Bacteria are usually more numerous at the edge than in the 

 center of the drop. 



B. Hanging Block. It is desirable occasionally to follow the 

 development of bacteria through several generations, to study the 

 germination of spores, or to examine special structures within the 

 bodies of individual organisms. The hanging-drop method is 

 unsuited for this purpose, which presupposes immobilization of the 

 organism. Hill 2 has invented an ingenious modification of the hang- 

 ing-drop method, the hanging block, which fulfils this requirement. 

 His directions for preparing it are: 



" Pour melted nutrient agar into a Petri dish to the depth of about 

 one-eighth or one-quarter inch. Cool this agar and cut from it a block 



1 Physiological salt solution is prepared by dissolving 8.5 grams NaCl in distilled 

 water 1000 c.c. 



2 Jour. Med. Research, March, 1902, vii, 202. 



