METHODS OF OBSERVING BACTERIA. 79 



glasses can never be too clean. It is best to immerse 

 them first in a strong mineral acid, then to wash them in 

 water, then in alcohol, then in ether, and keep them in 

 ether until they are to be used. Except that it some- 

 times cracks, bends, or fuses the edges of the glasses, a 

 better and more convenient method of cleaning them is to 

 wipe them as clean as possible, seize them in fine-pointed 

 forceps, pass them repeatedly through a small Bunsen 

 flame until it becomes greenish yellow, then slowly ele- 

 vate the glasses above the flame, so as to allow them to 

 anneal. This maneuvre removes the organic matter by 

 combustion. It is not expedient to use covers twice for 

 bacteriological work, though if well cleaned they may 

 subsequently be employed for ordinary microscopic ob- 

 jects. 



To return : After the material spread upon the cover 

 has dried, it must be fixed to the glass by immersion for 

 twenty-four hours in equal parts of absolute alcohol and 

 ether, or, as is much easier and more rapid, be passed 

 three times through a flame. Three is not a magic 

 number, but experience has shown that when drawn 

 through the flame three times the desired effect seems 

 best accomplished. The Germans recommend that a 

 Bunsen burner or a large alcohol lamp be used, that the 

 arm holding the forceps containing the cover-glass in- 

 scribe a circle a foot in diameter, and that, as each revo- 

 lution occupies a second of time, the glass be made to pass 

 through the flame 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. 



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 

 satiirated 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. 



