364 APPENDIX. 



sections being at once cut ; if they are to be imbedded 

 they should be washed with water and placed for a 

 quarter of an hour or more in 30 p.c., 50 p.c. and 

 70 p.c. alcohol When the tissues are to be kept 

 some time before cutting, they should be well washed 

 with water and then treated with alcohol as chromic 

 acid specimens are treated; of course if the tissue 

 has not been sufficiently hardened by the osmic acid, 

 its stay in dilute alcohol must be short. The tissues 

 become very much darker on staying in alcohol ; it is 

 not advisable to leave them long in this fluid before 

 sections are made. In osmic acid specimens the 

 nuclei are generally spherical and indistinct although 

 the nucleoli are not infrequently obvious. The 

 longer a tissue is left in osmic acid, the less easily 

 does it stain with carmine and other reagents. 



Osmic acid specimens which it is desired to preserve 

 should be mounted in dilute glycerine or in a con- 

 centrated aqueous solution of acetate of potash ; they 

 may, though with less satisfactory results, be cleared 

 and mounted in Canada balsam. 



Alcohol. 



Alcohol, besides its general use of completing the 

 hardening begun by other fluids, is sometimes used 

 alone. It coagulates the albumin in tissues and 

 thus renders them more opaque. 



It is generally best to place the piece of tissue for 

 half-an-hour to an hour in 75 p.c. alcohol, and 

 then to remove it to strong spirit (90 to 95 p.c.) ; 

 in some cases it is advantageous after the tissue 

 has been half-an-hour in strong spirit to transfer 

 it to absolute alcohol. Absolute alcohol when 



