CH. VII.-] PREPARATION OF REAGENTS. 177 



(A). 6% or thick collodion. It is made by mixing 50 cc. of sulphuric ether and 

 50 cc. of 95% alcohol and adding 6 grams of soluble cotton. If this is shaken re- 

 peatedly the solution will be complete in a day or two. 



(B). r,'-2% ox thin collodion. To prepare this 1% grams of soluble cotton are 

 added to 100 cc. of ether-alcohol (? 306). 



(C). }i% collodion or cementing collodion. To prepare it ^ths of a gram of 

 soluble cotton is added to iod cc. of ether-alcohol. 



As both ether and alcohol are very volatile it is necessary to keep the bottles 

 containing them well corked. 



ji 305. Eosin. — This is used mostly as a contrast stain with hematoxylin, which 

 is an almost purely nuclear stain. It serves to stain the cell-body, ground sub- 

 stance, etc., which would be too transparent and invisible with hematoxylin alone. 

 If eosin is used alone it gives a decided color to the tissue and thus aids in its 

 study ($ 135). Eosin is used in alcoholic and in aqueous solutions. A very satis- 

 factory stain is made as follows : 50 cc. of water and 50 cc. of 95% alcohol are 

 mixed and i-ioth of a gram of dry eosin added. 



The eosin is used after the hematoxylin in most cases (§ 280), and, as it is in 

 alcoholic solution, it may be washed off with 95% alcohol if the object is to be 

 mounted in balsam. If it is to be mounted in glycerin or glycerin jell}-, the excess 

 of eosin should be washed away with distilled water. 



I 306. Ether, Ether-Alcohol. — Sulphuric ether is meant when ether is men- 

 tioned in this book. For the ether-alcohol mentioned in $ 254, 304, etc., a mixture 

 of equal volumes of sulphuric ether and 95% alcohol is meant. 



\ 307. i-'arrant'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. 



\ 30S. Formaldehyde Dissociator. — This is composed of 5 cc. of a 40% solution 

 of formaldehyde in 995 cc. of water, to which 6 grams of common table salt 

 (sodium chlorid) have been added. That is, it is a T %% solution of formalde- 

 hyde in normal salt solution (§ 313). Formaldehyde as bought in the market is a 

 40% solution in water, and is called formol, formalin, formalose and formal, the 

 last name being the preferable one. For its use in isolating cells see § 245. (Gage 

 Micr. Bulletin and Sci. News, vol. XII. (1895), pp. 4-5). 



§309. Glycerin. — (A). One should procure pure glycerin for a mounting me- 

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



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 



