STAINING OF BACTERIA. 147 



fore, not to dilute till the liquid is transparent. If a cloud- 

 iness forms after a few days, the stain should be discarded 

 and a fresh solution prepared. The best results are ob- 

 tained when the dilute stain contains but little alcohol. If 

 the stock solution is incompletely saturated it will, on dilu- 

 tion, yield a transparent, feeble stain. 



The dilute anilin stain is prepared as follows: Some 

 of the saturated, alcoholic solution of the dye is placed in 

 a one-ounce bottle, and then diluted with three or four 

 parts of water. The bottle should be provided with a cork 

 through which passes a glass tube, the lower end of which 

 is slightly drawn out. This then serves as a pipette (Fig. 22, 

 p. 150). A drop of stain, as it leaves the pipette, should 

 be opaque. If this is not the case it is due, either, to the use 

 of an unsaturated stock solution, or, to excessive dilution. 



Simple staining. Place a small drop of water on a 

 cover-glass which is held between the thumb and forefinger 

 of the left hand. The drop of water should not be much 

 larger than a good sized pin-head. By means of the sterile 

 platinum wire, a portion of the growth is transferred to the 

 droplet of water. The latter should now be distinctly 

 cloudy. Sterilize the wire again by heating in the flame, 

 and, when cold, spread out the droplet of water over 

 the surface of the cover-glass. The water evaporates 

 rapidly, if the drop is small, and leaves a perfectly even 

 residue distributed over the entire surface. If the drop is 

 large, it will dry slowly and will leave unsightly "shore 

 lines." 



The evaporation of the water may be hastened by waving- the 

 cover-glass, to and fro, a few inches above a flame. Care must be 

 taken not to transfer too much material to the drop of water, for in 

 such a case the cover-glass on subsequent staining will be found to be 

 one mass of bacteria. 



Instead of making- the suspension on the cover-glass as indicated 

 above, the beginner will find it better to proceed as follows: Place one 



