38' THE AMERICAN MONTHLY [February, 



\x> forty or jifty times the volume of the specimeu. There is 

 only one exception — when the fixatives are powerful and ex- 

 pensive (osmic acid). 



AlcoJioL — Three concentrations are used : Ranvier's \ alcohol, 

 equally excellent for dissociating and for fixing. R is made of 

 alcohol, 36° C, one volume, with two volumes of water. Alcohol 

 of 90° is used daily in pathological anatomy. Portions of tissue 

 not exceeding a square centimetre are suspended in it by a thread. 

 A tissue capable of contracting — a bit of skin, for instance — 

 is spread on the bottom of a dry saucer, and while it is held with 

 two needles 90° alcohol is poured over it ; in a few moments the 

 needles are removed, for then the skin will not contract. This 

 rule should be observed, otherwise we are apt to have deformed 

 specimens or those which cannot be cut in any selected direction. 

 Although good preparations maybe made of specimens that have 

 been long in alcohol, it is better to take them out in from 24 to 

 48 hours. Absolute alcohol is rarely used as a fixing reagent. 

 Specimens should not be left in it for more than a day or they 

 will not stain well. The beginner should procure the one-third, 

 the 90°, and the absolute alcohol. The last is exceedingly useful 

 for many purposes 



Osmic Acid. — This is the fixative par excellence As- 



the cost is great and since the solution is unstable, the latter 

 should be made with certain precautions. A white or yellow 

 glass bottle is washed with sulphuric acid. (R is not the light 

 but organic matter that reduces the solution. The bottle should 

 have a glass stopper.) Rinse with distilled water to remove every 

 trace of acid. The little tube containing the osmic acid is well 

 washed in alcohol, nicked with a file, and glass and acid are put 

 into the bottle. Add 50 c. c. of distilled water. A i per cent, 

 solution is generally used. R' the cleansing has been perfect the 

 solution will remain clear. It is used as a vapor or in solution. 



Fixation by the vapor is easiest and most efficacious for small 

 objects. With free elements — blood, for instance — invert the slide 

 over a wide-mouthed bottle in which is a little osmic acid. In a 

 few minutes tlie fixation is perfect, and the preparation can be 

 stained and mounted. Larger objects (not exceeding i mm. 

 square) and membranes are fastened by pins to a cork which then 

 stops a wide-mouthed bottle containing i c. c. of osmic acid. 

 Fixation is here slower. The acid should be allowed to act for 

 from 10 to iK, minutes or even longer. Wash rapidly in distilled 

 water, stain or keep in alcohol for sectioning. When a tissue is 

 placed directly in the i per cent, solution, the specimen, we re- 

 peat, should be as small as possible (i mm.). In a bottle with 

 ground-glass stopper place 2 c. c. of the solution. The tissues 

 remain tliere for a variable time ; 6 hours are usually sufficient. 

 The specimen should be carefully washed in many waters till all 

 odor has been removed Uo to 12 hours) ; preserve in 70"^ alcohol. 



