SPECIAL METHODS. I/? 



d. Picric Acid, C 6 H 2 .(NOu) 3 .OH. 



303. Picric acid is used mostly in a concentrated aqueous 

 or alcoholic solution. Its action for 24 hours is generally 

 sufficient. Before staining it must be carefully washed out, 

 for which purpose running water is especially useful (cf. 

 35). But in many cases it is better to wash with alcohol, 

 in which picric acid is more readily soluble than in water. 



e. Picro-sulphuric Acid. 



304. Picro-sulphuric acid may be prepared, according to^ 

 the recipe proposed by Mayer, by mixing 100 volumes of 

 water and two of concentrated sulphuric acid, then shaking 

 up with it as much picric acid as will dissolve, and finally 

 diluting the whole with three times its volume of water. 

 Picro-sulphuric acid has the advantage over pure picric acid 

 that it is more easily washed out. It has been much used 

 with the lower organisms. 



C. Chromic Acid, H 2 CrO 4 . 



305. Chromic acid has been used with the best results for 

 fixing algae, especially in a \% aqueous solution. Its action 

 for a few hours is always sufficient for these plants ; but 

 with larger tissues it is better to allow the medium to act 

 for 24 hours. Before staining, the chromic acid must always 

 be well washed out, for which running water is best ( 35). 

 Overton recommends (I, 10) for this purpose a weak aqueous 

 solution of sulphurous acid. This makes objects fixed in 

 chromic acid fit for staining with haematoxylin and carmine 

 in a few minutes. 



Chromic acid has the disadvantage of often causing, espe- 

 cially in tissues rich in tannin, the formation of precipitates 

 which hinder observation. 



306. Finally, it may be observed that, according to Vir- 

 chow (I), objects fixed with chromic acid should be brought 

 in contact with alcohol only in the dark, before the com- 

 plete removal of the acid, since their power of staining is 

 lessened by the formation of a precipitate in the light. In 



