MOUNTING AND PRESERVING OBJECTS. 79 



glass, which should be cemented at the edges, and gummed 

 over all a strip of colored or lithographed paper, in which 

 an aperture has been made with a punch. 



Mounting in Balsam or Dammar Varnish is suitable for 

 a very large proportion of objects. It increases the trans- 

 parency of many structures, tilling up interstices and cavi- 

 ties, and giving them a smooth, beautiful appearance. 

 Very delicate tissues, as fine nerves, etc., are rendered in- 

 visible by it, and require other fluids, as glycerin. 



Before mounting in balsam, the object should be thor- 

 oughly dry, otherwise a milky appearance will result. It 

 should then be placed in oil of cloves or of turpentine to 

 remove greasiuess and increase the transparency. A clean 

 slide, warmed over a spirit-lamp or on a hot plate, should 

 then have a little balsam placed on its centre, and the 

 object carefully lifted from the turpentine is put into the 

 balsam and covered with another drop. The slide should 

 then be gently warmed, and any air-bubbles pricked with 

 a needle-point or drawn aside. The thin glass cover should 

 be warmed and put on gently, in such a way as to lean 

 first on one edge and fall gradually to a horizontal posi- 

 tion. The slide may be warmed again, and the superflu- 

 ous balsam pressed from under the cover by the pressure 

 of a clean point upon it. 



If the object is full of large air-spaces and is not likely 

 to be injured by heat, the air may be expelled by gently 

 boiling it in the balsam on the slide. If the object be one 

 which will curl up, or is otherwise injured by heat, the 

 air-pump must be resorted to. A cheap substitute for the 

 air-pump may be made by fitting a piston into a tolerably 

 wide glass tube closed at one end. The piston should 

 have a valve opening outwards. The preparation in bal- 

 sam may be placed at the bottom of the tube, and a few 

 strokes of the piston will exhaust the air. 



To fill a deep cell with Canada balsam, it may be well 

 to fill it first with turpentine and place the specimen in 



