520 PREPARATION, MOUNTING, AND COLLECTION OF OBJECTS 



When used, the jelly must be liquefied by gentle warmth, and it is 

 useful to warm both the slide and the cover-glass previously to 

 mounting. This takes the place of what was formerly known as 

 Dean's medium, in which honey was used to prevent the hardening 

 of the gelatin. i 



A. For objects which would be injured by the small amount of 

 heat required to liquefy the last-mentioned medium, the glycerin and 

 gum medium of Mr. Farrants will be found very useful. This is 

 made by dissolving four parts (by weight) of picked gum arabic in four 

 parts of cold distilled water, and then adding two parts of glycerin. 

 The solution must be made without the aid of heat, the mixture being 

 occasionally stirred, but not shaken, whilst it is proceeding ; after it 

 has been completed the liquid should be strained (if not perfectly 

 free from impurity) through fine cambric previously well washed out 

 by a current of clean cold water ; and it should be kept in a bottle, 

 closed with a glass stopper or cap (not with cork), containing a small 

 piece of camphor. The great advantage of this medium is that it 

 can be used cold, and yet soon viscifies without cracking ; it is well 

 suited to preserve delicate animal as well as vegetable tissues, and in 

 most cases increases their transparence. 



Of late years glycerin has been largely used as a preservative, 

 either alone, according to the method of Dr. Beale, or diluted 

 with water, or mixed with gelatinous substances. It is much more 

 favourable to the preservation of colour than most other media, and 

 is therefore specially useful as a constituent of fluids used for 

 mounting vegetable objects in their natural aspects. It has also the 

 property of increasing the transparence of animal structures, though 

 in a less degree than resinous substances, and may thus be advan- 

 tageously employed as a component of media for mounting objects 

 that are rendered too transparent by balsam or dammar. Two 

 cautions should be given in regard to the employment of glycerin : 

 first, that, as it has a solvent power for carbonate of lime, it should 

 not be used for mounting any object having a calcareous skeleton ; 

 and second, that, in proportion as it increases the transparence of 

 organic substances, it diminishes the reflecting power of their 

 surfaces, and should never be employed, therefore, in the mounting of 

 objects to be viewed by reflected light, although many objects 

 mounted in the media to be presently specified are beautifully 

 shown by * dark-ground ' illumination. 



1. A mixture of one part of glycerin and two parts of 

 camphor-water may be used for the preservation of many vegetable 

 structures. 



2. For preserving soft and delicate marine animals which are 

 shrivelled up, so to speak, by stronger agents, the Author has found 

 a mixture of one part of glycerin and one of spirit with eight or ten 

 parts of sea- water the most suitable preservative. 



3. For preserving minute vegetable preparations the following 

 method, devised by Hantsch, is said to be peculiarly efficient : A mix- 

 ture is made of three parts of pure alcohol, two parts of distilled water, 

 and one part of glycerin ; and the object, laid in a cement-cell, is 

 to be covered with a drop of this liquid, and then putaside under a bell- 



