1 48 PA THOGENIC BA CTERIA . 



that as 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 material is ready for the stain. Every 

 laboratory should be provided with several stock-solutions 

 of the more ordinary dyes. These stock-solutions are 

 saturated alcoholic solutions made by adding 25 grams 

 of the dye to 100 c.cm. of alcohol. Of these it is well to 

 have fuchsin, gentian violet, and methylene blue always 

 made up. The stock-solutions will not stain, but are the 

 standards for the manufacture of the working stains. 

 For ordinary staining an aqueous solution made in a 

 simple manner is employed. A small bottle is nearly 

 filled with distilled water, and the stock-solution is added, 

 drop by drop, until the color becomes just sufficiently in- 

 tense to prevent the ready recognition of objects through 

 it. For exact work it is probably best to give these 

 stains a standard composition, using 5 c.c. of the satu- 

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

 watery solution possesses the power of readily penetrat- 

 ing the dried protoplasm of the bacterium, taking the 

 stain with it. Alcohol does not have this power. 



As in the process of staining the cover is apt to slip 

 from the fingers and spill the stain, it is well to be pro- 

 vided with cover-glass forceps (Fig. 8), which hold the 



Fig. 8. — Stewart's cover-glass forceps. 



glass in a firm grip and allow of all manipulations with- 

 out danger to the fingers or clothes. The ordinary in- 

 struments are entirely unfitted for the purpose, as capil- 

 lary attraction draws the stain between the blades and 

 makes certain the soiling of the fingers. Sufficient stain 

 is allowed to run from a pipette upon the smeared side 



