DEHYDRATION AND CLEARING 93 



wards, on a ledge till dry, and then the slides are stored in a wide 

 stoppered jar till needed. The floating out is performed as before. 

 The albuminised side of the slide is easily recognised by the fact that if 

 it is breathed on, the breath does not condense on it. The great 

 advantage of this method is that the section is fixed after twenty to 

 thirty minutes' drying at 37 C. If the tissue has been hardened in any 

 of the bichromate solutions and embedded in paraffin, this or some 

 corresponding method of fixing the sections on the slide must be used. 



Preparation of Paraffin Sections for Staining. Before stain- 

 ing, the paraffin must be removed from the section. This is 

 best done by dropping on xylol out of a drop bottle. When the 

 paraffin is dissolved out, the superfluous xylol is wiped off with 

 a cloth and a little absolute alcohol dropped on. When the 

 xylol is removed the superfluous alcohol is wiped off and a 

 little 50 per cent methylated spirit dropped on. During these 

 procedures sections must on no account be allowed to dry. 

 The sections are now ready to be stained. Deposits of crystals 

 of corrosive sublimate often occur in sections which have been 

 fixed by this reagent. These can be removed by placing the 

 sections, before staining, for a few minutes in equal parts of 

 Gram's iodine solution (p. 99) and water, and then washing out 

 the iodine with methylated spirit. 



To save repetition we shall in treating of stains suppose that, 

 with paraffin sections, the above preliminary steps have already 

 been taken, and further that sections cut by a freezing microtome 

 are also in spirit and water. 



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

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

 Dry films after being stained are washed in water, dried and 

 mounted in xylol balsam ivet Jilms and sections must be 

 dehydrated, 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 placed on a slide is partially dried by 

 draining with fine blotting-paper. 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. 

 Balsam as ordinarily supplied has often an acid reaction, and 



