STAINING METHODS. 271 



foe either freshly prepared or at least freshly filtered, or in dilute 

 alcoholic solutions. In order to prepare these latter we make 

 saturated alcoholic solutions of the stains, preferably in drop- 

 bottles, and add them drop by drop to a considerable bulk of 

 distilled water, which should be at least tenfold that of the 

 alcoholic solution. Watery methyl violet, gentian violet, or 

 fuchsin solution must always be freshly prepared, while dilute 

 methylene-blue solution keeps. Bismarck brown or Vesuvin 

 changes in alcohol, and must therefore be kept in a watery solu- 

 tion, but must always be filtered before use ; but the other stains 

 are to be preferred to them. The bacteria in a fluid medium are 

 spread out on a cover-glass, in the thinnest possible layer, and 

 allowed to dry, exactly as is described above, at room temperature 

 and protected from dust. If the fluid contains albuminous bodies 

 or mucilage, these, after complete drying of the preparation, must 

 be fixed, which can be effected either by laying the cover-glass for 

 several days in absolute alcohol, or at once by high temperature. 

 This latter method, which is very rapid and convenient, can be 

 carried out by holding the dried cover- glass by means of forceps 

 (best of all the flat-ended, bent forceps with crossed legs, so as 

 to open with pressure, and known as " cover-glass forceps "), and 

 passing it about three times moderately quickly through the flame 

 of a Bunsen burner, or a strong spirit-lamp flame, the bacteria- 

 covered side being held upwards. Too long action of the flame 

 injures the staining properties of the bacteria, so that it is desir- 

 able not to too greatly prolong it. The preparation is stained by 

 spreading upon the cover- glass, prepared in this kind of way 

 (and which must in all cases be air-dry), some drops of the stain, 

 and allowing it to act for a few seconds, though in some circum- 

 stances longer action is necessary. With bacteria which take the 

 stain badly, warming the staining fluid is an assistance. For this 

 we hold the cover-glass in the flame with the staining solution 

 upon it, till the fluid begins to steam. After staining, the cover- 

 glass is rinsed in distilled water, its clear side dried with a linen 

 rag, and the preparation may then be at once examined in water ; 

 or, after rinsing, and drying the clear side, the preparation may be 

 dried at room temperature, cleared by means of a drop of turpen- 

 tine, xylol, oil of cedar, or oil of bergamot placed upon it, and 

 examined in this. If the bacteria are found to be over stained, a 

 part of the colour can be removed if absolute alcohol is allowed to 

 act upon it for a sufficient time. The same result can be attained 



