360 APPENDIX. 



"When the tissue is -well hardened, the hardening agent 

 should be extracted from it by placing it in alcohol, 

 first dilute 30 p.c. to 50 p.c. then stronger 50 p.c. to 

 75 p.c. ; it should then be preserved in 70 to 95 p.c. 

 alcohol. 



Since chromic acid, potassium bichromate, etc., are 

 precipitated by alcohol in the light, tissues hardened 

 in these reagents, after being washed with water arid 

 placed in alcohol, should be kept in the dark, until 

 required for cutting, or until the alcohol, frequently 

 renewed, no longer becomes yellow. Many tissues 

 when well hardened may be left a day or more in 

 water without injury before placing in dilute alcohol. 

 Tissues hardened in picric acid are best placed an 

 hour or two only in the dilute alcoholic solutions, 

 and then transferred to 70 to 95 p.c. alcohol, which 

 may be renewed as often as it becomes coloured. 



The tissues should be placed in a flat short bottle to 

 harden, rather than in a narrow high one; they should 

 be kept cool during the first day or two, after this it 

 is of less importance, though usually advantageous. 



Preparation of Solutions. 



It is generally advisable to make of any given sub- 

 stance a solution of the maximum strength likely to 

 be required, and to dilute this when weaker solu- 

 tions are needed. If the stronger solution contains 

 X p.c. of the substance and it is required to make 



a weaker solution of y p.c, add — 1 c.c. of water 



y 



to each c.c. of the stronger solution. 

 It is rather better to use distilled water in making up 



