140 



BACTERIOLOGY. 



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 re- 

 sults 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. 31), and when used are dropped upon 



FIG. 31. 



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 

 accomplished by the addition to the solutions of small 

 quantities of alkaline substances or by dissolving the 

 staining materials in strong watery solutions of either 



