156 Methods of Observing Bacteria 



from apex to base three times. This is supposed to be exactly 

 the requisite amount of heating. The rule is a good one for 

 the inexperienced. 



Inequality in the size of various flames may make it de- 

 sirable to have a more accurate rule. Novy * suggests that 

 so soon as it is found that the glass is so hot that it can no 

 longer be held against the finger it is sufficiently heated for 

 fixing. 



After fixing, the preparation is ready for the stain. Every 

 laboratory should be provided with stock solutions of the 

 ordinary dyes. These are saturated alcoholic solutions made 

 by adding 25 grams of the dye to 100 c.c. of alcohol. Of 

 these it is well to have fuchsin, gentian violet, and methyl- 

 ene-blue always made up. The stock solutions will not 

 stain, but form the basis of the staining solutions. For 

 ordinary staining an aqueous solution is employed. A small 

 bottle is nearly filled with distilled water, and the stock 



Fig. 22. Stewart's cover-glass forceps 



solution added, drop by drop, until the color becomes just 

 sufficiently intense to prevent the ready recognition of ob- 

 jects through it. For exact work it is probably best to give 

 these stains a standard composition, using 5 c.c of the 

 saturated alcoholic solution to 95 c.c. of water. Such a 

 watery solution possesses the power of readily penetrating 

 the dried cytoplasm of the bacterium. 



Cover-glasses are apt to slip from the fingers and spill the 

 stain, so when using them it is well to be provided with 

 special forceps (Fig. 22), which hold the glass in a firm grip 

 and allow of all manipulations without danger of soiling 

 the fingers or clothes. The ordinary sharp-pointed forceps 

 are unfit for the purpose, as capillary attraction draws 

 the stain between the blades and makes certain the soiling 

 of the fingers. In using the special forceps the glass should 

 not be caught at the edge, but a short distance from it, as 

 shown in the cut. This altogether prevents capillary at- 

 *" Laboratory Work in Bacteriology ' 1899 



