22 INTRODUCTION. 



partially guarded against by fastening to the upper surface of 

 the slide, near its two ends, square pieces of pasteboard a little 

 thicker than the cell itself. 



106. Glycerin-jelly, a mixture of glycerin with pure gelatin, is 

 liquid at the temperature of boiling water, and solidifies again 

 on cooling. Any specimen which is not injured by being slightly 

 heated can be mounted satisfactorily in the jellj', provided it 

 is first thoroughly- saturated with gl^xerin. But this precaution 

 is by no means necessar}' in all cases. 



107. A drop of the melted jell3-, fi'ee from air-bubbles, is 

 placed on the slide (a fragment of the solid jellj- can be melted 

 on the slide if preferred), the specimen placed therein, and the 

 cover-glass, previously moistened slightl_y on the under side with 

 glycerin, is carefullj- laid on, and the preparation now allowed 

 to cool. When the jelly is again hard, a varnish or cement ring 

 may be placed around the edge of the cover to hold it in place. 

 Asphalt-cement is apt to impart to the jell}- a dark tinge, which 

 may sooner or later spoil the mount, and hence the colorless 

 varnishes are better. 



108. The edge of the jelly may be lightly touched with a 

 strong solution of a chromate, for instance, bichromate of potas- 

 sium, and exposed for a while to light. This renders the jelly 

 insoluble, and firmly sets it. 



109. The following are among the best formulas for making 

 this useful mounting-medium : — 



One part of pure gelatin, three parts of water, and four of 

 glycerin (Schacht, quoted by Dippel). Nordstedt uses the same 

 proportions, and advises the addition of a small piece of cam- 

 phor or a drop of carbolic acid, to prevent moulding. 



One part of gelatin is soaked in six parts of water for two 

 hours, seven parts of glycerin are added, and one per cent of 

 carbolic acid is added to the whole. The mass is heated for 

 fifteen minutes, with constant stii-ring, and then filtered through 

 glass-wool. All the ingredients must be absolutely pure (Kaiser, 

 Bot. Centrbl., 1880, p. ih). 



The proportions employed in the second formula, but without 

 the addition of the carbolic acid, give a clearer jell3' ; and it has 

 not been apt to mould, especially if the cork of the bottle con- 

 taining it be wrapped in a thin piece of linen, which has been 

 dipped in dilute carbolic acid. 



110. Canada halsam. This is used either (1) alone, or (2) in 

 solution. In either case the specimen must be free from water, 

 and permeated by some liquid easily miscible with the balsam. 



