28 BOTANICAL MICROTECHNIQUE. 



After fixing, the objects are transferred to alcohol. This 

 is accomplished by first adding a drop of 10-20$ alcohol, 

 which causes no collapse, and then placing the preparation 

 in a close chamber saturated with alcohol vapor, to bring 

 about a gradual concentration of the alcohol. For this 

 purpose a flat crystallizing dish may be used, its upper edge 

 being ground so as to be hermetically sealed by a greased 

 glass plate. Its bottom is covered with absolute alcohol 

 and a stand to hold the preparations is placed in it. The 

 latter may be made by simply bending down the ends of 

 some strips of sheet zinc. According to Overton, the cover- 

 glass with the objects is placed, with its wet side upward, 

 on a piece of elder pith, about 3 mm. high and of a diameter 

 less than that of the cover-glass, which, in its turn, rests on 

 an ordinary slide. In this apparatus, which must be pro- 

 tected from sudden changes of temperature and especially 

 from direct insolation, the 20$ alcohol on the cover-glass 

 becomes almost absolute alcohol in a few hours, by diffusion 

 through the air. When this is accomplished, a drop of a 

 dilute solution of celloidin is placed on the cover-glass and 

 evenly spread over its upper surface by tipping it backward 

 and forward. It is of advantage to make the celloidin film 

 as thin as possible, since thicker films both separate more 

 easily and render the subsequent manipulations much more 

 difficult. Therefore pretty thin solutions of celloidin must 

 be used. A suitable one may be prepared by diluting the 

 ordinary officinal solution of celloidin with ten times its bulk 

 of a mixture of equal parts of alcohol and ether.* 



As soon as the celloidin no longer flows evidently, the 

 -whole cover-glass (or slide) is placed in 80$ alcohol, wet side 

 vup. Here the celloidin film becomes so hard in a few 

 ^minutes that the objects can be placed in suitable staining 

 fluids without being washed away. A long-continued action 

 of alcohol of more than 00% is to be avoided, as it dissolves 



* [The thinnest of the celloidin solutions recommended for use in imbed- 

 ding (cf. 4ga) may be diluted with twice its own bulk of the alcohol-ether 

 mixture for this purpose.] 



