1 78 BOTANICAL MICROTECHNIQUE. 



the alcohol used for washing, a precipitate is also formed in 

 '.the light ; but when this is removed by filtering the fluid 

 scan again be used for washing. 



r/. Chrom-formic Acid. 



307. Rabl recommends (I, 215) a mixture of 200 grams 

 of \% chromic acid with four or five drops of concentrated 

 formic acid for fixing nuclear figures. It must be freshly 

 prepared before use, each time, and should act for 12-24 

 hours. It must be well washed out with water before 

 staining. 



O. Osmic Acid, OsO 4 . 



308. To fix sections or small objects, the vapor of osmic 

 acid may best be used by placing the objects in a drop of 

 water on a cover-glass or slide and bringing the drop over 

 the mouth of a bottle containing a i% or 2% solution of 

 osmic acid. Killing and fixing take place almost instantly. 

 For fixing larger pieces of tissue \<f> osmic acid may be used 

 .and may act for several hours without harm. 



Osmic acid, which is indisputably one of our best fixing 

 -media, has the disadvantage of producing brown or black 

 precipitates with very various substances. But in most 

 cases these precipitates may be removed subsequently with- 

 out injury to the protoplasmic structure, and for this use 

 hydrogen peroxide is best adapted. Overtoil (I, 11) recom- 

 mends for this purpose a mixture of one part of commercial 

 peroxide with 10-25 parts of 70-80$ alcohol. I have also 

 observed that, even after the use of the concentrated com- 

 mercial solution of peroxide, which decolorizes at once, 

 especially on gentle warming, the karyokinetic figures re- 

 main quite sharp and unchanged in microtome sections. 



i. Chrom-osmic-acetic Acid. 



309. Mixtures of chromic acid, osmic acid, and acetic 

 acid have been used with the best results in the study of 

 the karyokinetic figures, especially by Flemming. This 

 author has used solutions of very different strengths ; but 



