Cn. X] REAGENTS AND THEIR PREPARATION 365 



in the mounting medium when shellac is used for sealing. A small 

 amount of lamp-black well rubbed up in very thin shellac and filtered 

 is good to darken the shellac. 



§ 589. Silvering. — Intercellular substance stains brown or black 

 with nitrate of silver. Use \ or \ % aqueous solution on fresh 

 tissue. Stain in the silver for 1 or 2 minutes; then expose to light in 

 water till brown. Fix in 82% alcohol or 5% formaldehyde. One 

 may stain afterward with hematoxylin for the nuclei; mount in 

 glycerin, glycerin jelly, or in balsam. 



§ 590. Sudan III for fat. — Sudan III, or azo-benzene-azo-/3-naph- 

 thol, was introduced by Daddi into histology in 1896 (Arch. Ital. de 

 Biologie, t. 26, p. 142), as a specific stain for fat. As it is soluble in 

 all forms of fat and oils and in xylene, alcohol, etc., it is impossible 

 to mount specimens in balsam after staining. As the fat of tissues is 

 removed by the reagents used in the paraffin and collodion methods 

 (see Ch. XI), only teased, free-hand, or frozen sectioned material fresh 

 or fixed in some non-fat dissolving fixer can be used (Midler's fluid 

 and 5 v ' formaldelhyde are excellent). The tissues cut free-hand or 

 with the freezing microtome or teased can then be stained with a 

 saturated alcoholic solution of the Sudan. It stains all fat a brilliant 

 red. Preparations can be preserved in glycerin or glycerin jelly. 

 This stain is largely used in pathology. 



Daddi used the substance to feed animals and thus to stain the fat 

 which was laid down in the body while the Sudan was fed. 



The fat in the body already deposited remains unstained. This 

 substance then serves to record the deposit of fat in a given period. 

 In 1907 Dr. Oscar Riddle fed Sudan to laying hens, and the fat in the 

 layers of yolk laid down during the feeding was stained red (Science, 

 XXVII, 1908, p. 495). For staining the yolks of hen's eggs the hen 

 may be fed doses of 20 to 25 milligrams of the Sudan. Eggs so colored 

 hatch as usual, and the chick in utilizing the colored yolk stains its 

 body-fat pink (Susanna P. Gage). 



§ 591. Table Black. — During the last few years an excellent method 

 of dying wood with anilin black has been devised. This black is 

 lusterless, and it is indestructible. It can be removed only by scrap- 

 ing off the wood to a point deeper than the stain has penetrated. 



