SPECIAL STAINING SOLUTIONS. 131 



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 junt 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. 30), and when used are dropped upon 

 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. 



Eiick of bottles for staining solutions. 



For certain bacteria which stain only imperfectly 

 with these simple solutions it is necessaiy 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 

 aniline oil or carbolic acid, instead of simple water — in 

 other words, by employing mordants with the stains. 



Of the solutions thus prepared which may always be 

 employed upon bacteria that show a tendency to stain 



