GENERAL METHODS. 43 



sible. The sections are placed directly in this fluid, and, 

 since a thin layer of gelatine is soon formed at the edge of 

 the cover-glass by the evaporation of water, the preparation; 

 may be sealed after a short time with maskenlack or an 

 alcoholic solution of sealing-wax. Many stains, such as 

 those with iodine green or carmine, are well preserved in 

 this medium. 



60. Enclosure in Air. Ash skeletons, crystals easily solu- 

 ble in water, and the like may be often best preserved sim- 

 ply dry. To protect them from dust it is also necessary in 

 such cases to cover the objects with a cover-glass. This 

 may be attached to the slide by wax or paraffine around its 

 edges or even with gummed paper. 



61. The observation of crystalline precipitates and the 

 like is best conducted in the air by ordinary light ; while in 

 polarized light the interference colors appear most pure on 

 enclosure in a strongly refractive medium like Canada bal- 

 sam. Instructive preparations of both kinds may be pre- 

 pared by placing on the slide a drop of Canada balsam so 

 small that it occupies only a part of the space beneath 

 the cover-glass, leaving a part of the crystals in air. To 

 exclude destructive agencies so far as possible, the edge of 

 the cover-glass may then be surrounded with paraffine or 

 wax. 



62. Sealing Media. Of the numerous sealing media pro- 

 posed by various authors may be mentioned here first the 

 so-called "gold-size," which is well adapted for glycerine 

 and glycerine-gelatine preparations. Since the method of 

 preparing it is quite elaborate, it is best to obtain it ready- 

 prepared (e.g., from Dr. G. Griibler, Leipzig). 



For glycerine-gelatine preparations Canada balsam, asphalt 

 varnish, and maskenlack N. Ill are also well adapted. The 

 cover-glass cement containing amber, recommeded by Hey- 

 denreich, affords a very trustworthy medium ; but it should 

 not be colored with eosin, as was the case with a prepara- 

 tion formerly furnished by Dr. G. Griibler, because this 

 gradually goes over into the glycerine-gelatine and may* 

 cause an unpleasant staining of the preparation. 



