148 BACTERIOLOGY. 



or two large drops of water on a slide and touch the wire, laden with 

 the growth, into this water until a cloudy suspension results. Touch 

 a straight sterile wire to this liquid and transfer the droplet that 

 adheres to a cover-glass. The thin film dries almost immediately 

 and is not likely to contain too many bacteria. Any number of 

 preparations can be made from the original suspension. 



A sliminess of the liquid, and a tendency of the bacteria to 

 clump, can be overcome by gently heating the suspension over a flame. 



As soon as the water has evaporated the material is 

 "fixed" to the cover-glass by means of heat. For this pur- 

 pose the cover-glass is taken up in the forceps, specimen 

 side up, and is touched, from above downward, once or twice, 

 to a Bunsen, or alcohol flame. This little operation re- 

 quires judgment and care. The cover-glass must become 

 heated sufficiently to fix the specimen, so that it will not 

 wash off when subsequently treated with the dye and with 

 water. On the other hand, too much heating' will destroy 

 the organism and render it incapable of taking 1 up the dye. 

 The best rule to follow is to touch the cover-glass, once or 

 twice, to the flame and then to bring it immediately into 

 contact with the end of the finger. If the cover-glass is so 

 hot that the finger must be withdrawn at once, it indicates 

 that the specimen is fixed and that it is not necessary to 

 heat any more. 



The cover-glass, with the specimen side still turned up, 

 is held in the forceps and a drop or two of water is placed 

 on the specimen. Then, two or three drops of the dilute 

 gentian violet, or fuchsin are added, and allowed to act for 

 for one-quarter to one-half minute. The cover-glass is then 

 washed perfectly clean of the dye, by being held under a 

 tap, or by rinsing in one or two glasses of water. It is 

 then touched edgewise to a piece of filter or blotting paper 

 in order to drain off the excess of water, and is then placed 

 on the paper with the specimen side turned up. By gentle 

 rotation of the cover-glass, the lower surface becomes per- 

 fectly dry. This can be easily done if the paper is sup- 

 ported on the tip of the index finger. 



