76 GENERAL MICROBIOLOGY 



tion, this is an indication of currents in the liquid which 

 have been induced by the liquid touching the side of the 

 concavity, by the drop being too large, by improper mixing, 

 or by air currents; this fault may be remedied by thoroughly 

 mixing the bacteria in the drop with the straight needle 

 or by resealing the cover-glass upon the slide. 



Apparatus. Clean cover-glasses; clean concave slides; 

 platinum loop; straight platinum needle; Bunsen burner; 

 distilled water; cover-glass forceps; melted paraffin or 

 vaselin if preparation is to be sealed permanently. 



Method. 1. With a platinum loop place four small 

 drops of water about the edge of the depression of the con- 

 cave slide. 



2. For cultures: 



In liquid media. 



(a) With a sterile platinum loop transfer a portion 

 of the culture to the center of a clean cover- 

 glass. 



On solid media. 



(a) With a sterile platinum loop place a small drop 

 of water or physiological salt solution in the 

 center of a clean cover-glass. 



(6) With a sterile platinum needle transfer a minute 

 portion of the culture to the drop of water so 

 that only the faintest cloudiness appears. 



3. Quickly invert the cover-glass over the depression in 

 the concave slide and gently depress the margin on the water 

 until the chamber is sealed air tight. The hanging drop 

 must not touch the bottom of the concavity. Note the 

 illustration. If it is desired to keep the hanging drop 

 longer than five to ten minutes, it may be sealed with 

 paraffin as with the adhesion culture, or with vaselin. 



The drop must remain over the center of the concavity. 

 If the drop touches the side of the concavity, the hanging 

 drop as such is destroyed and it will be necessary to remake 

 the preparation. If pathogenic organisms are used, both 



