362 REAGENTS AND THEIR PREPARATION [Ch. X 



except for special purposes. Those like that shown in fig. 203 may 

 be had for about $3 for 10,000. 



§579. Mercuric chlorid (HgCl 2 ). — Mercuric chlorid 7^ grams; 

 sodium chlorid 1 gram; water 100 cc. The solution is facilitated 

 by heating in an agate dish. Fix fresh tissue in this 2 to 24 hours. 

 Then transfer to 67 % alcohol a day or more and then to 82 % alcohol. 

 Tissues fixed in mercuric chlorid deteriorate; hence it is better to 

 imbed them soon after they are fixed. Crystals of mercury are 

 removed from the sections by the use of iodized alcohol (§576). 



§ 580. Methylene blue, alkaline. — Methylene blue 2 grams; 95% 

 or absolute alcohol 50 cc. ; distilled water 450 cc. ; 1 % aqueous caustic 

 potash 5 cc. This stain works best after a fixer containing mercuric 

 chlorid, like Zenker's fluid (see § 563 for eosin in alcohol). 



§ 581. Miiller's fluid. — Potassium dichromate 2§ grams; sodium 

 sulphate, 1 gram; water 100 cc. This is one of the oldest fixers. It 

 must act a long time, two weeks to 10 or 12 weeks. This longer time 

 is for nervous tissue to be stained for the myelin. Lately this fixer 

 has been combined with mercury (see Zenker's fluid § 592). Before 

 putting the tissue into 67 % alcohol it is washed out in running water 

 for 24 hours. 



Miiller's fluid 10 cc; normal salt solution 90 cc. forms an excel- 

 lent dissociator for epithelia, etc. (§ 514). 



§ 582. Neutral red. — This is used especially for staining living 

 animals. It is used in very weak solutions: T \ T gram red; 1000 cc. 

 of water. Put a few cubic centimeters of this solution into the vessel 

 containing the live animal, or animals. Infusoria stain quickly, 10 

 to 20 minutes or less. Vertebrates may require a few days. Try 

 it on infusoria by adding a drop of the red to several drops of the 

 infusion containing the infusoria. Be sure that there are many 

 animals present. Watch them under the microscope and the color 

 will be seen appearing in the granules of the infusoria. Then one 

 may cover and study with a high power (see § 521). 



§ 583. Nitric acid, HNO3. — This is employed for dissociation 

 (nitric acid dissociator: water 80 cc, nitric acid 20 cc); as a fixer, 

 especially for chick embryos in the early stages (water 90 cc. ; nitric 

 acid, 10 cc), and as a delcacifier (nitric acid 3CC.; 67 % alcohol 100 cc). 



