52 HARDENING AGENTS. 



BEWSKT found that nerve-cells hardened in 10 per cent, solution of sugar of 

 lead were admirably preserved. See her "Inaug.-Diss." in Mitth. d. naturf. 

 Ges. Bern., 1887, and Zeit.f. wiss. Mik., iv, 3, 1887, p. 387. 



88. Alcohol. When used alone, alcohol is inferior as a 

 hardening agent to most of the reagents discussed above ; 

 but when judiciously employed to complete the action of a 

 good fixing agent, it renders most valuable services. 90 to 

 95 per cent, is the -most generally useful strength. Weaker 

 alcohol, down to 70 per cent., is often indicated. Absolute 

 alcohol is seldom advisable. You ought to begin with weak, 

 and proceed gradually to stronger, alcohol. Large quanti- 

 ties of alcohol should be taken. The alcohol should be 

 frequently changed, or the tissue should be suspended near 

 the top of the alcohol, in order to have the tissue constantly 

 surrounded with pure spirit (the water and colloid matters 

 extracted from the tissue falling to the bottom of the vessel). 

 Many weeks may be necessary for hardening large specimens. 

 Small pieces of permeable tissue, such as mucous membrane, 

 may be sufficiently hardened in twenty-four hours. 



89. Iodine may be used in combination with alcohol, and render service 

 through its great penetrating power. See the method of BETZ, post, Part II. 



90. Pyridin. Pyridin has been lately recommended as a hardening 

 agent (by A. DE SOUZA). It is said to harden, dehydrate, and clear tissues 

 at the same time. They may be stained after hardening by anilin dyes dis- 

 solved in the pyridin, or passed through water and stained by the usual pro- 

 cesses. It is said to harden quickly, and to give particularly good results 

 with brain. See Comptes Rendus hebd. de la Soc. de Biologic, 8 ser., t. iv, 

 No. 35, p. 622; Zeit.f. wise. Mile., v, i, 1888, p. 65 ; Journ. Eoy. Mic. Soc., 

 1888, p. 1054. 



