146 



BACTERIOLOGY. 



The solutions with which the staining is accom- 

 plished are made from these stock solutions in the fol- 

 lowing way : 



An ordinary test-tube of about 13 mm. diameter is 

 three-fourths filled with distilled water and the concen- 

 trated alcoholic or watery solution of the dye is then 

 added, little by little, until one can just see through 

 the solution. It is then ready for use. Care must be 

 taken that the color does not become too dense. The 

 best results are obtained when it is just transparent as 

 viewed through a layer of about 12 to 14 mm. thick. 



These represent the staining-solutions in everyday 

 use. They are kept in bottles supplied with stoppers 

 and pipettes (Fig. 35), and when used are dropped upon 



FIG. 35. 



Rack of bottles for staining-solutions. 



the preparation to be stained. After remaining upon 

 the preparation for from twenty to thirty seconds they 

 are washed off in water, and the preparation can then 

 be examined. 



For certain bacteria which stain only imperfectly 

 with these simple solutions it is necessary to employ 

 some agent that will increase the penetrating action of 

 the dyes. Experience has taught us that this can be 



