CH. X] REAGENTS AND THEIR PREPARATION 357 



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 i cc., water 19 cc. 537.) See also 

 516 (i) for the formaldehyde dissociator. 



569. 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, i 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 mount- 

 ing 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 517-518. 



570. Glycerin jelly for microscopic specimens. Soak 25 grams 

 of the best dry gelatin in cold water in a pyrex or agateware dish. 

 Allow the water to remain until the gelatin is softened. It usually 

 takes about half an hour. When softened, as may be readily deter- 

 mined 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 albu- 

 men, 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 flocculent precipitate which seems to gather all fine particles 

 of dust, etc., leaving the gelatin perfectly 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 thor- 



