Acetic Acid. 



Glacial acetic acid is a valuable clearing agent. In 2 per cent, 

 solutions it is good for clearing up the cell contents and in study- 

 ing the nucleus or protoplasmic structure. In strong solutions it 

 dissolves the cell contents and makes the cell wall clear. It is also 

 used in testing for oxalate crystals which are insoluble in it, but 

 which dissolve without effervescence in HC1, while carbonate crystals 

 dissolve with effervescence in both. 



Alum. 



Alum is employed as a mordant in various staining processes, 

 as, for example, in Frey's haematoxylin. It is also used to render 

 more visible cells that have become too transparent by treating 

 with KOH. 



Ammonia. 



Strong aqueous ammonia is sometimes used in preference to 

 KOH where the action of the latter would be too violent. If tissue 

 with thick walled cells be placed in nitric acid and then in ammonia, 

 the middle lamella of the cells will be colored yellow. Ammonia is 

 also used in the preparation of certain stains and in Schweizer's 

 reagent for dissolving cellulose without essentially changing its 

 composition. ' 



Anilin Chloride. 



Aniliu chloride is used as a test for lignin, one of the con- 

 stituents of wood. The sections to be treated are placed in a dilute 

 solution until they are thoroughly saturated. They then assume a 

 pale yellow color which is much deepened upon the addition of 

 HC1. This is Hoehnel's test for lignin. 



Argentic Nitrate. 



A dilute alkaline solution of silver nitrate when fresh is used 

 as a test for living protoplasm. The aldehyde which is contained 

 in the living protoplasm precipitates metallic silver free in the solu- 

 tion and colors the protoplasm dark. Dead protoplasm is not 

 affected in any way. 



