82 MOUNTING AND LABELING. 



PREPARATION OF MOUNTING MEDIA. 



\ 188. Glycerin. — One should procure pure glycerin for a mounting medium. 

 It needs no preparation, except in some cases it should be filtered through filter 

 paper or absorbent cotton to remove dust, etc. 



For preparing objects for final mounting, glycerin 50 cc, water 50 cc, form a 

 good mixture. For many purposes the final mounting in glycerin is made in an 

 acid medium, viz., Glycerin 99 cc, Glacial acetic or formic acid, 1 cc. 



By extreme care in mounting and by occasionally adding a fresh coat to the 

 sealing of the cover-glass, glycerin preparations last a long time. They are liable 

 to be very disappointing, however. In mounting in glycerin care should be taken 

 to avoid air-bubbles, as they are difficult to get rid of. A specimen need not be 

 discarded unless the air-bubbles are large and numerous. 



§ 189. Glycerin Jelly. — Soak 25 grams of the best dry gelatin in cold water in a 

 small agate-ware dish. Allow the water to remain until the gelatin is softened. It 

 usually takes about half an hour. When the gelatin is softened, as may be readily 

 determined by taking a little in the fingers, pour off the superfluous water and 

 drain well to get rid of all the water that has not been imbibed by the gelatin. 

 Warm the softened gelatin over a water bath and it will melt in the water it has 

 absorbed. Add to the melted gelatin about 5 cc. of egg albumen (white of egg) ; 

 stir it in well and then heat the gelatin in the water bath for about half an hour. 

 Do not heat above 75 or So C, for if the gelatin is heated too hot it will be 

 transformed into meta-gelatin and will not set when cold. The heat will coagu- 

 late the albumen and form a kind of floculent precipitate which seems to gather 

 all fine particles of dust, etc., leaving the gelatin perfectly clear. After the gela- 

 tin is clarified it should be filtered through a hot filter and mixed with an equal 

 volume of glycerin and 5 grams of chloral hydrate and shaken thoroughly. If it 

 is allowed to remain in a warm place (i. e. , in a place where the gelatin remains 

 melted) the air-bubbles will rise and dissapear. 



In case the glycerin jelly remains fluid or semi-fluid at the ordinary temperature 

 (i8°-2o° C), the gelatin has either been transformed into meta-gelatin by too high 

 temperature or it contains too much water. The amount of water may be lessened 

 by heating at a moderate temperature over a water bath in an open vessel. 

 This is a very excellent mounting medium. Air-bubbles should be avoided in 

 mounting as they do not disappear. 



\ 190. Farrant's Solution. — Take 25 grams of clean, dry, gum arabic ; 25 cc. of 

 a saturated aqueous solution of arsenious acid ; 25 cc. of glycerin. The gum ara- 

 bic is soaked for several days in the arsenic water, then the glycerin is added and 

 carefully mixed with the dissolved or softened gum arabic. 



This medium retains air-bubbles with great tenacity. It is much easier to 

 avoid than to get rid of them in mounting. For the method of mounting in this 

 see \ 173. 



\ 191. Canada Balsam, Balsam of Fir. — This is one of the oldest and most sat- 

 isfactory of the resinous media used for mounting microscopical preparations. 

 Sometimes it is used in the natural state, but experience has shown that it is bet- 

 ter to get rid of the natural volatile constituents. A considerable quantity, half a 

 liter or more, of the natural balsam is poured into shallow plates in layers abont 1 

 or 2 centimeters thick, then the plates are put in a warm, dry place, on the back 

 of a stove or on a steam radiator, and allowed to remain until the balsam may be 

 powdered when it is cold. This requires a long time, the time depending on the 

 temperature and the thickness of the layer of balsam. 



