96 MICROSCOPIC METHODS. 



over-stained films and sections. More of the acid may, of 

 course, be added if necessary. 



Hot water also decolorises to a certain extent, and over- 

 stained films can be readily decolorised by placing a drop 

 of water on the film and heating gently over a flame. 



When preparations have been sufficiently decolorised by 

 an acid, they should be well washed in tap water, or in dis- 

 tilled water with a little lithia carbonate added. 



The methods embracing the use of a stain with a mord- 

 ant, and a decoloriser, are very numerous, and we can only 

 enumerate the best of them. 



Dehydration and Clearing. It is convenient, first of all, 

 to indicate the final steps to be taken after a specimen is 

 stained. We have already described the mounting of film 

 preparations. Sections must be dehydrated, cleared, and 

 then mounted in xylol balsam. 



Dehydration is most commonly effected with absolute 

 alcohol (it is economical to use first the cheaper methylated 

 spirit, and then to finish up with alcohol, but this complica- 

 tion is hardly necessary). Alcohol, however, sometimes 

 decolorises the stained organisms more than is desirable, 

 and therefore Weigert devised the following method of 

 dehydrating and clearing by aniline oil, which, though it 

 may decolorise somewhat, does not do so to the same 

 extent as alcohol. As much as possible of the water being 

 removed, the section is placed in aniline oil ; or if it has 

 been cut in paraffin, some aniline oil is placed on the 

 section, and the slide moved to and fro. The section is 

 dehydrated and becomes clear. The process may be 

 accelerated by heating gently. The preparation is then 

 treated with a mixture of two parts of aniline oil and one 

 part of xylol, and then with xylol alone, after which it is 

 mounted in xylol balsam. 



Sections stained for bacteria should always be cleared^ at 

 least finally, in xylol, for the same reason that xylol balsam 

 is to be used for mounting films, viz. that it dissolves out 

 aniline dyes less readily than such clearing reagents as 

 clove oil, etc. Xylol, however, requires the previous de- 



