io6 MICROSCOPIC METHODS. 



here preferable, as they do not readily overstain. In the 

 case of such fluids, if the histological elements also claim 

 attention it is best first to stain the cellular protoplasm 

 with a one to two per cent watery solution of eosin (which 

 is an acid dye), and then to use a blue which will stain the 

 bacteria and the nuclei of the cells. In the case of films 

 made from urine, where there is little or no albuminous 

 matter present, the bacteria may be imperfectly fixed on the 

 slide, and are thus apt to be washed off. In such a case it 

 is well to modify the staining method. A drop of stain 

 is placed on a slide, and the cover-glass, film-side down, 

 lowered upon it. After the lapse of the time necessary for 

 staining, a drop of water is placed at one side of the cover- 

 glass and a little piece of filter- paper at the other side. 

 The result is that the stain is sucked out by the filter- 

 paper. By adding fresh drops of water and using fresh 

 pieces of filter-paper, the specimen is washed without 

 any violent application of water, and the bacteria are not 

 displaced. 



For the general staining of films a saturated watery 

 solution of methylene-blue will be found to be the best 

 stain to commence with. 



The Use of Mordants and Decolorising Agents. In 

 films of blood and pus, and still more so in sections of 

 tissues, if the above methods are used, the tissue elements 

 may be stained to such an extent as to quite obscure the 

 bacteria. Hence many methods have been devised in 

 which the general principle may be said to be (a) the use 

 of substances which, while increasing the staining power, 

 tend to fix the stain in the bacteria, and (^) the subsequent 

 treatment by substances which decolorise the overstained 

 tissues to a greater or less extent, while they leave the 

 bacteria coloured. The staining capacity of a solution may 

 be increased 



(a) By the addition of substances such as carbolic acid, 

 aniline oil, or metallic salts, all of which probably act as 

 mordants. 



