GENERAL METHODS. 13 



by osmotic action and is especially adapted for small objects. 

 As is shown in the accompanying Fig. 4, in which the bell- 

 shaped cover that closes the vessel is not 

 shown, this consists chiefly of two cyl- 

 inders, broadened at the top and placed 

 one within the other, their lower ends 

 being closed by a membrane which per- 

 mits osmotic exchange between water 

 and alcohol. Schulze recommends for 

 this purpose a thin writing-paper known 

 as " Postverdruss," * which is glued to the 

 ground lower edge of the cylinder. 



In the inner cylinder are placed the ~ 



' FIG. 4. Dehydrating ves- 



objects to be dehydrated in very dilute, sel - After F. E. Schuize. 

 about 10$, alcohol ; in the outer cylinder is placed a small 

 quantity of stronger, about 50$, alcohol ; and in the vessel 

 containing the cylinders is absolute alcohol, which is kept 

 water-free by a layer of anhydrous copper sulphate on the 

 bottom of the vessel. For complete dehydration a period 

 of twenty-four hours is always sufficient. Further, the 

 rapidity of the osmotic interchange may be largely regu- 

 lated by changes of the differences in level between the 

 different fluids. With less sensitive objects one may find one 

 cylinder sufficient, and then the dehydration can be accom- 

 plished in a few hours. 



17. According to the method proposed by Overton (I, 12), 

 dehydration may be conducted by placing the objects first 

 in 10$ glycerine. In this, objects which have been fixed 

 never suffer collapse ; and living ones may first be killed by 

 osmic-acid fumes (cf. 308). The preparations are then left 

 exposed to the air without a cover-glass, but protected from 

 dust by a bell-jar. The solution of glycerine is thus so con- 

 centrated by the evaporation of the water that finally a 

 transfer to alcohol is possible without any collapse. 



* [The American papers manufactured under the name of "parchment- 

 paper "and the finer grades of the so-called " Overland paper " serve the pur- 

 pose well. Best of all is true parchment; and chamois-skin has also been 

 recommended.] 



