DEHYDRATION AND CLEARING. 107 



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

 ammonium carbonate, in weak solution. 

 (<:) 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), alcohol (either methylated spirit or absolute 

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

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

 also various oils, e.g., aniline, clove, etc. In most cases 

 about thirty drops of acetic acid in a bowl of water will 

 be sufficient to remove the excess of stain from over-stained 

 films and sections. More of the acid may, of course, be 

 added if necessary. 



Hot water also decolorises to a certain extent ; 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 mor- 

 dant, 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 after being stained must be de- 

 hydrated, cleared, and then mounted in xylol balsam. 



Dehydration is most commonly effected with absolute 

 alcohol. 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 



