1. =Potassium Ferrocyanid Test for Iron.= 1. Stain sections in lithium carmine for several hours. 2. Differentiate in acid alcohol, stopping short of the desired complete differentiation of the nuclei. 3. Wash in water. 4. Saturated solution of potassium ferrocyanid 1-3 hours. 5. Acid alcohol until iron-pigment becomes blue (½-12 hours). Complete differentiation of nuclei. 6. Wash in water. 7. Dehydrate in absolute alcohol. 8. Clear in xylol; mount in balsam. Hæmosiderin is blue (Berlin blue); nuclei are red. Lithium carmine may be used after the iron-test, if desired. 2. =Ammonium Sulphide Test for Iron.= 1. Fix in alcohol; imbed; cut. 2. Treat sections with yellow ammonium sulphide for 5-60 minutes. 3. Wash quickly in water. 4. Dehydrate in absolute alcohol. 5. Clear in xylol; mount in balsam. Stain with lithium carmine either before or after the reaction with ammonium sulphide. Iron is grayish-black to black. 3. =Combined or Masked Iron.= 1. Treat tissues with Bunge’s fluid (95 per cent alcohol 95 cc., 25 per cent hydrochloric acid 10 cc.) for 1-2 hours at 50-60°C., until inorganic iron is all removed. 2. Place tissues in acid alcohol (sulphuric acid 4 cc. in 100 cc. alcohol 95 per cent.) 5. Wash sections in acid alcohol, then pure alcohol, and finally in distilled water. 6. Transfer to ammonium sulphide (5-60 minutes) or to potassium ferrocyanid and O.5 HCl for 5 minutes. 7. Wash in water. 8. Counterstain in eosin or safranin; wash; dehydrate in absolute alcohol; clear in cedar-oil; mount in benzene balsam. Keep preparations in the dark. 4. =Staining of Chromophilic Cells.= 1. Fix in a chromic solution. In this the chromophilic cells become yellow or brown. 2. Stain in polychrome methylene blue; the cells become grass-green in color. 5. =Tests for Silver, Lead and Mercury.= Use ammonium sulphide as for iron. Black sulphides are formed. 6. =Test for Copper.= Treat with potassium ferrocyanid and hydrochloric acid; copper gives a dark yellow-brown coloration. =XXIII. PSEUDOMUCIN.= It is not precipitated by acetic acid. It has a greater affinity for the diffuse stains than mucin, and gives weaker metachromatic reactions. =XXIV. REGENERATION AND REPAIR.= For the staining of mitoses, cell granules and cell-inclusions see methods given above. See also methods for staining of epithelium, reticulum, neuroglia, etc. XXV. URIC ACID AND PURIN BASES:— 1. =Courmont and Andre’s Method.= 1. Fix in absolute alcohol; imbed; cut. 2. Treat sections with 1/100 ammonia solution or very weak sodium hyposulphite solution. 3. Transfer to 1/100 silver nitrate solution. 4. Wash. 5. Develop with a photographic developer. 6. Wash in water; stain with hæmalum and eosin; dehydrate; clear in xylol; balsam. Uric acid and xanthin or purin bases appear as black granules.