102 VETERINARY BACTERIOLOGY 



Quite as effective an observation may be made in many cases 

 by placing a drop of the culture upon a glass slide and dropping 

 a cover-glass upon it, using care to include a few air-bubbles. 

 A film of liquid sufficiently thick for the free movement of the bac- 

 teria will remain between the two glasses. The edges of the air- 

 bubbles furnish a convenient object upon which to focus. 



STAINING METHODS 



Bacteria as well as the pathogenic protozoa are generally so 

 transparent when examined in a living condition that the details 

 of their morphology can be made out only with difficulty. It is 

 customary to stain these organisms with various anilin dyes 

 which render them distinctly visible. 



The stains used in biological work are, for the most part, known 

 as anilin dyes, because they are derivatives of anilin, C 6 H 5 NH 2 . 

 They are grouped as acid or basic, depending on whether the acid 

 radical or the base possesses the tinctorial powers. Fuchsin, 

 for example, is a basic stain, while ammonium picrate is an acid 

 stain. The basic stains are the more useful in the study of bacteria; 

 the acid stains are sometimes used as counterstains, particularly 

 for tissues in which the organisms may be embedded. The 

 anilin dyes are of all the colors of the rainbow. The most com- 

 monly used are gentian-violet, methylene-blue, thionin blue, 

 fuchsin, and Bismarck-brown. 



Mordants. Anything which will cause a stain to penetrate an 

 organism better or which causes it to set is termed a mordant. 

 For example, carbolic acid or anilin added to certain stains makes 

 them more intense. A solution of iodin in potassium iodid, a 

 mixture of tannic acid and iron sulphate, and many other solu- 

 tions are used under various conditions as mordants. 



Formulas of Some of the Commonly Used Stains. There are a 

 few stains which find constant use in the laboratory. The formulas 

 of these will be given. There are, in addition', a great many others 

 which have special applications. 



Loffler's methylene-blue: 



Saturated alcoholic solution of methylone-blue 15 c.c. 



Solution of potassium hydrate (1 : 1000) 50 c.c. 



