SPECIAL METHODS. 179 



the following, which have both proved very good with 

 plant-cells, may be mentioned here. The first, dilute mixt- 

 ure contains .25$ of chromic acid, .\% of osmic acid, and .1% 

 of acetic acid. The more concentrated mixture is prepared 

 from 15 volumes of \% chromic acid, 4 volumes of 2% osmic 

 acid, and one volume (or less) of glacial acetic acid. 



Both mixtures must be carefully washed out with water. 

 Blackening due to the osmic acid may be removed with 

 hydrogen peroxide (cf. 308). 



For preserving objects fixed with the above mixture, 

 Flemming (III, 687, note) recommends a mixture of water, 

 alcohol, and glycerine, in about equal parts. This affects 

 their staining capacity less than pure alcohol. 



K. Corrosive Sublimate, HgCl 2 . 



310. Corrosive sublimate is usually best used in concen- 

 trated alcoholic solution, though the concentrated aqueous 

 solution often does well. Action for a few hours is always 

 sufficient for complete fixing, but it may be left on the 

 objects without harm for 24 hours. Alchohol to which 

 enough iodine has been added to give a dark brown solution 

 may be used for washing out the sublimate. If it is not 

 wholly washed out, needle-shaped or sphaerite-like crystals 

 of sublimate may be seen in the preparation and may easily 

 deceive beginners. But where the sublimate has not been 

 wholly removed before imbedding in parafftne, it may subse- 

 quently be washed out, even from microtome sections, with 

 the iodine-alcohol. 



If it is desirable to avoid alcohol in washing out sublimate, 

 the mixture, proposed by Haug (I, 13), of two parts' tinc- 

 ture of iodine, one part potassium iodide, 50 parts glycerine, 

 and 50 parts water may be used. It should be renewed 

 until no further decolorization occurs. 



I will remark that objects fixed with sublimate must not 

 be touched with iron forceps or the like before it is wholly 

 washed out, since globules of mercury are thus easily pro- 

 duced within the objects. In this case forceps with platinum 

 or horn points may be used. 



