52 THE MICROSCOPIC PREPARATION. 



fore the balsam cools the preparation is covered with a large cover-glass, 

 warmed by passing several times through the flame. 



Kopsch (96) places specimens in a solution composed of 10 c.c. 

 of formalin (40% formaldehyd) and 40 c.c. of a 3.5% solution of potas- 

 sium bichromate. For objects 2 c.c. in size 50 c.c. of the fluid are em- 

 ployed ; but if the specimens be large, the mixture must be changed in 

 twelve hours. At the end of twenty-four hours this fluid is replaced by a 

 fresh 3.5% potassium bichromate solution, and the specimens are then 

 transferred to a 0.75% solution of silver nitrate (after two days, if the 

 tissue be the liver or stomach ; and after from three to six days, if retina 

 or central nervous system). After this treatment the objects are car- 

 ried over into 40% alcohol and, finally, into absolute alcohol, imbedded 

 as rapidly as po'ssible. and cut. The sections are mounted in balsam 

 without a cover-glass. 



PREPARATION OF PERMANENT SPECIMENS. 



The resinous media used in the final mounting of preparations are 

 Canada balsam and damar. 



Canada Balsam. Commercial Canada balsam is usually dissolved in 

 turpentine ; it should be slowly evaporated in a casserole and then dissolved 

 in xylol, toluol, or chloroform, etc. The proper concentration of the solu- 

 tion is found with a little experience. A thick solution penetrates the 

 interstices of the section with difficulty, and usually contains air-bubbles 

 which often hide the best areas of the preparation, and can only be re- 

 moved with difficulty by heating over a flame. Thin solutions, on the 

 other hand, have also their disadvantages ; they evaporate very quickly, and 

 the empty space thus created between the cover-slip and slide must again 

 be filled with Canada balsam. This is best done by dipping a glass rod 

 into the solution and placing one drop at the edge of the cover-slip, 

 whereupon the fluid spreads out between the cover-slip and slide as a 

 result of capillary attraction. Canada balsam dries rather slowly, the 

 rapidity of the process depending upon the temperature of the room. To 

 dry quickly, the slides may be held for a few moments over a gas or 

 alcohol flame, or they may be placed in a warm oven, where the prepara- 

 tions become so dry in twenty-four hours that they can be examined with 

 an oil-immersion lens. The oil used for this purpose should be wiped 

 away from the cover-slip after examination. This can only be done, with- 

 out moving the cover-slip, when the balsam is thoroughly dry and holds 

 the cover-slip firmly in place. 



Damar. Damar is dissolved preferably in equal parts of oil of tur- 

 pentine and benzin. It has the advantage of not rendering the prepara- 

 tion as translucent as Canada balsam. Otherwise it is used as the latter. 



Clearing Fluids. Since alcohol does not mix with Canada balsam 

 or damar, an intermediate or clearing fluid is used in transferring objects 

 from the former into the latter. Xylol, toluol, carbol -xylol (xylol, 3 parts; 

 carbolic acid, i part), oil of bergamot, oil of cloves, and oil of origanum 

 are ordinarily used. The process is somewhat simpler where sections are 

 fixed to the slide. Xylol is dropped onto the surface of the slide, or better, 

 the whole preparation is placed for a few minutes in a vessel containing 

 xylol until the diffusion currents have ceased (which may be seen with 

 the naked eye). The slide is then taken out, tilted to allow the xylol to 

 run off, wiped dry around the object with a cloth, and placed upon the 



