MORDANTS AND DECOLORISERS. 



95 



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 (b) 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 ac 

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

 mordants. 



(b) by the addition of alkalies, such as caustic potash 

 ammonium carbonate, in weak solution. 



(c) By the employment of heat. 



(d) By long duration of the staining process. 



As decolorising agents we use chiefly mineral acids 

 (hydrochloric, nitric, sulphuric), vegetable acids (especially 

 acetic acid), and alcohol (either methylated spirit or absolute 

 alcohol), or a combination of spirit and alcohol, e.g., methyl- 

 ated spirit with a drop or two of hydrochloric acid added. 

 In most cases about thirty drops of acetic acid in a bowl of 

 water will be sufficient to remove the excess of stain from 



