USE OF MORDANTS AND DECOLORISERS. 99 



Some bacteria, e.g. typhoid, glanders, take up the stains rather 

 slowly, and for these the more intensive stains, red or violet, 

 are to be preferred. 



Films of fluids from the body (blood, pus, etc.) can be gen- 

 erally stained in the same way, and this is often quite sufficient 

 for diagnostic purposes. The blue dyes are here preferable, as 

 they do not readily overstain. Should overstaining occur it is 

 easily remedied by decolorising for a few seconds in glacial acetic 

 acid, i-iooo, and removing the acid by thoroughly washing in 

 water. In the case of such fluids, if the histological elements 

 also claim attention, it is best first to stain the cellular proto- 

 plasm 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 (1^) 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. 



