APPENDIX 297 



generally used, but it causes great shrinking, especially of the 

 outer parts of the tissue. 



The shrinking is less if the tissue is placed in 70 to 75 p.c. 

 alcohol for some hours to a day ; transferred to 90 or 95 p.c. for 

 a day or two, and then to absolute alcohol. When well hardened, 

 the tissue may be kept in 75 p.c. alcohol. 



Chromic acid. Make a 1 p.c. solution; from this make 

 2 p.c. and other solutions. Tissues to be hardened in chromic 

 acid -2 p.c. are left in it about a week. (Cp. p. 91.) If sections 

 of tissues hardened in chromic acid or in chromic salts do not 

 stain well, place them for about 3 hours in acid alcohol, and wash 

 with alcohol before staining. 



Formol or formaline is a 40 p.c. aqueous solution of form- 

 aldehyde. A solution containing 10 p.c. of formol, i.e. 4 p.c. of 

 formaldehyde, is the most generally useful. It has great pene- 

 trating power and fixes rapidly, and without external shrinking. 



It is perhaps best to leave a piece of tissue of moderate size 

 not more than a day, to wash thoroughly and pass through 

 alcohols as given above. Otherwise the staining is apt to be 

 diffuse. 



The vapour of formol may be used, as the vapour of osmic 

 acid, to fix films, or small fragments of tissue. 



Mercuric chloride. A saturated solution in water or in 

 *75 p.c. salt solution is used. It is a good hardening agent for 

 glands, and epithelium of skin and cornea. Tissues should not as 

 a rule be left in it more than one to two days. They are then 

 washed in running water (cp. p. 296) and passed through alcohols, 

 kept for a week or more in 95 p.c. alcohol, then transferred to 

 75 p.c. alcohol till required. 



Unless the piece of tissue is small, the mercuric salt will not 

 be wholly removed by this process; to facilitate its removal, a 

 drop or two of iodine dissolved in potassic iodide is added to the 

 stronger alcohols, until the fluid is no longer decolourized. Then 

 the alcohol is renewed. 



The salt is similarly removed from the tissues when any of 

 the fluids given below which contain mercuric chloride are used. 



