62 THE MICROSCOPIST. 



Valentin's double knife (Fig. 33) is used for soft tissues 

 where only a moderate degree of thinness is needed. The 

 blades should be wet, or the section made under water. 



Soft substances often require hardening before sections 

 can be made. The most simple and best method is that 

 of freezing, by surrounding the specimen with a freezing 

 mixture, when it may be cut with a cold knife. Small 

 pieces of tissue may be hardened in absolute alcohol, fre- 

 quently renewed. Chromic acid, in solution of one-fourth 

 to two per cent., is often used for animal tissues, or bichro- 

 mate of potash of the same strength. A solution of one- 

 fifth to one-tenth per cent, of perosmic acid or of chloride 

 of palladium is also recommended. 



Soft tissues often require imbedding in a concentrated 

 solution of gum or of wax, spermaceti, or paraffin tem- 

 pered with oil. In this case sections may be made readily 

 by means of a section-cutter. For imbedding in wax, etc., 



FIG. 33. 



Valentin's Knife. 



the specimen must be hardened in alcohol, then treated 

 with oil of cloves or turpentine, and the section should be 

 mounted in Canada balsam or Dammar varnish. 



Sections of hard substances, or of those imbedded, are 

 often made by machines invented for the purpose. One 

 of the simplest is (Fig. 34) an upright hollow cylinder, 

 with a kind of piston, pushed upwards by a fine screw. 

 The upper end of the cylinder carrying the specimen ter- 

 minates in a flat table, along which a sharp knife or flat 

 razor is made to slide. At one side of the tube is a 

 binding-screw for holding the specimen steady. A sec- 



