HANGING-DROP PREPARATION 47 



diameter and of No. 1 thickness are used. Cover-glasses 

 may be round or square, as preferred. They are cleaned 

 with sulphuric acid and potassium bichromate, and 

 kept, when clean, in alcohol, from which they are removed 

 with forceps, and either wiped with a clean rag or passed 

 through the flame to burn away the alcohol. To obtain 

 a satisfactory preparation, the cover-glass must be free 

 from grease. On a clean cover-glass or slide a minute 

 droplet of water or other liquid can be evenly spread, 

 whereas in the presence of grease it will run into pools. 

 A satisfactory film cannot then be expected, and there is a 

 probability of its washing off during manipulation. A 

 cover-glass should never be wiped with a circular or rotary 

 motion, but the wiping rag should be gently drawn across 

 it, taking care, in the case of a cover-glass, that the 

 pressure on both sides of it is equal. 



Hanging-Drop Preparation. Motility is most evident 

 in young cultures, and broth or agar cultures of twenty- 

 four hours old are most suitable. A hollow-ground slide 

 is passed through a Bunsen flame and then laid on a 

 clean bit of filter-paper, excavated side up. A ring of 

 vaseline is painted round the outside margin of the 

 excavation with a match-end. A clean cover-glass is 

 passed through the flame, laid on a clean flat surface, and 

 a droplet of a broth culture placed on the centre (when 

 a culture from a solid medium is used, a droplet of sterile 

 broth or water is placed in the centre of the sterile cover- 

 glass and inoculated with a minute trace of growth). 

 The excavated slide is lifted, turned over, and gently 

 lowered over the cover-glass, so that the drop of culture is 

 in the centre of the well and the ring of vaseline forms 

 a seal. The slide is now turned over so that the cover- 

 glass is uppermost, and placed under the microscope. For 

 organisms of the size of the typhoid bacillus a -^ objective 

 is suitable, but when a higher power is to be used, the 

 edge or centre of the droplet should first be centred with 

 a low-power objective. The light should be diminished, 

 either by nearly closing the diaphragm or by lowering 

 the condenser. Neither the Brownian movement (p. 2) 

 nor movement due to currents in the fluid should be 

 confounded with motility, in which the movement of 

 individual bacteria is independent, progression taking 

 place in different directions. Conversely, motile bacteria, 



