5 



Mounts and Mountants 



The final mounting of an object or section for microscopical examination 

 consists of cementing it between a slide and coverslip in a medium or 

 "mountant" which will preserve it permanently and retain it in a sufficiently 

 transparent condition for study. There are two kinds of mountants: (1) Those 

 which are miscible with water and to which objects may be transferred directly 

 and (2) those which are not miscible with water and which require that the 

 object be prepared by dehydration and clearing as described in the last chapter. 



All too little attention is paid to the water-soluble mountants which are 

 really much more suitable for mounting many objects than is the balsam 

 usually employed. Stained objects, of course, cannot be mounted in these 

 aqueous media, but a large number of small invertebrates, particularly arthro- 

 pods, make better preparations in gum media than they do in balsam. 



There are two types of gum media: (1) Those of relatively low index of 

 refraction which do not render the objects placed in them very transparent 

 and (2) those of a very high index of refraction for use in circumstances where 

 a transparency almost equivalent to a balsam mount is required. The two 

 most useful of the low index of refraction media are: 



Farrants' Medium: 



Water 40 ml. 



Gum acacia 40 Gm. 

 Glycerin 20 ml. 



Phenol 0.1 Gm. 



The original formula used a saturated solution of arsenous oxide as a pre- 

 servative, but phenol is much better. One may use, in point of fact, any 

 preservative to inhibit the growth of fungi. This medium is rather difficult to 

 make up, for it is almost impossible to obtain a sample of gum acacia which 

 is not contaminated with pieces of bark, sand, and dirt. A 50 per cent solution 

 of this material, even when diluted with the glycerin, is very difficult to filter, 

 and it is much better to secure this medium from a supplier of materials than 

 to make it up oneself A medium which avoids this difficulty by using syn- 

 thetic materials is: 



35 



