154 EXPERIMENTS ON MOTILITY 



twenty-four hour growth on agar does quite well (a younger one is perhaps better, 

 but flagella can be shown for a week or fortnight or more). 



[The following solutions are required : 



(a) The mordant. 



2 per cent, aqueous solution of osmic acid, ... 1 part. 



20 per cent, aqueous solution of tannin, 3 or 4 parts. 



(b) Silver nitrate solution. 



Crystals of silver nitrate, - - 1 gram. 



Distilled water, 100 c.c. 



(c) Reducing solution. 



2 per cent, aqueous solution of gallic acid, - - 1 part. 



Ammonia fort., 1 -------- 1 ., 



Mix immediately before use. 



[1. Place the mordant on the film for one or two minutes or less (time 

 unimportant). 



[2. Wash in tap-water thoroughly. Shake off as much water as possible. 



[3. Place a few drops of the silver nitrate solution on the film for a few 

 seconds or longer. 



[4. Shake off the excess of silver solution. 



[5. Allow one drop of the reducing solution to fall on the middle of the film 

 from a pipette. A wave spreads away from the centre to each end of the 

 slide. As soon as the film is seen standing out clearly and black (a few 

 seconds), wash off in tap- water. 



[6. Pour another drop or two of the silver solution on to the film and leave 

 for half a minute or so. 



[7. Wash in tap-water. Blot. Dry over a flame. The preparations fade 

 rapidly if mounted in balsam or cedar- wood oil.] 



SECTION IV. METHODS OF STUDYING THE MOTILITY OF 

 MICRO-ORGANISMS. 



Closely connected with the morphological study of the flagella of micro- 

 organisms is the investigation of their motility. 2 Motile organisms can make 

 their way through porous substances such as sand or a filter composed of 

 porcelain or siliceous earth. The time occupied in traversing a given thick- 

 ness of sand, etc., will vary according as to whether the organism is actively 

 or feebly motile. 



These observations are utilized for determining whether or no an organism 

 is motile, for separating motile from non-motile species, for determining the 

 relative motility of different strains of the same organism, and even for 

 creating, by a process of selection, races which are endowed with exceptional 

 powers of movement. 



A. Cambier has drawn attention to the property possessed by the typhoid 

 bacillus of traversing the walls of porous structures, and has suggested that 

 this property might be made use of in attempting the isolation of the organism. 



A porous porcelain bougie is placed in a large test-tube, and both the 

 bougie and the test-tube are half-filled with broth ; the tube is plugged 

 with wool and the whole apparatus autoclaved. When cool, the broth in 

 the bougie is sown with a culture of the typhoid bacillus. After incubating 

 for a few hours at 37 C. the broth in the tube surrounding the bougie will 

 be distinctly cloudy, and this is due to the fact that the typhoid bacillus 



1 [It is essential that the solution of ammonia be the strongest obtainable.] 

 z Vide also Chap. VII., Dark-ground illumination. 



