CH. IX} PREPARATION OF REAGENTS 277 



For fixing tissues and embryos a 5% solution is good (Formalin 1 cc. , 

 -water 7 cc, \ 377). A common fixer is 10 cc. formalin, 90 cc. water. This is 

 frequently called 10% formalin, it is however only 4% formaldehyde. 



Tissues may stay in this indefinitely. Small pieces are fixed within an 

 ■hour. Before hardening in alcohol and imbedding, wash out the formalin in 

 running water half an hour, then harden a day or more in 67% and 82% 

 alcohol. 



For preserving nitric acid dissociated muscle a 2% formaldehyde solution 

 is good. (Formalin 1 cc, water 19 cc. (S 377.) See also \ 399 (1) for the for- 

 maldehyde dissociator. 



\ 407. Glycerin. — (A.) One should have pure glycerin for a mounting 

 medium. It needs no preparation, unless it contains dust when it should be 

 filtered through filter paper or absorbent cotton. 



To prepare objects for final mounting, glycerin 50 cc, water 50 cc, forms 

 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 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, however, unless the air-bubbles are large and numerous. 

 See also Congo glycerin \ 397. 



\ 408. Glycerin Jelly for Microscopic Specimens. — 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 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 about 5 cc. of egg 

 albumen, white of egg ; stir it well and then heat the gelatin in the water bath 

 for about half an hour. Do not heat above 75° or 80° C, for if the gelatin is 

 heated too hot it will be transformed into meta-gelatin and will not set when 

 cold. Heat coagulates the albumen and it forms a kind of floculent precipitate 

 which seems to gather all fine particles of dust, etc., leaving the gelatin per- 

 fectly clear. After the gelatin is clarified, filter through a hot flannel filter 

 and mix with an equal volume of glycerin and 5 grams of chloral hydrate and 

 shake thoroughly. If it is allowed to remain in a warm place (i. c, in a place 

 where the gelatin remains melted) the air-bubbles will rise and disappear. 



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

 perature (i8°-2o° C), the gelatin has either been transformed into meta-gela- 

 tin by too high a 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 an excellent mounting medium. Air-bubbles 

 should be avoided in mounting as they do not disappear. 



I 409. Glycerin Jelly for Anatomic Preparations. — Specimens prepared 

 hy the Kaiserling method or other satisfactory way may be permanently pre- 



