188 



JSXPEBIMENTS WITH PLANTS 



chimney is now inverted, the enlarged base is then 

 carefully warmed and melted sealing-wax poured in to 

 the depth of an inch or more. A slightly larger disk 

 of rubber is now prepared by fitting a rubber stopper 

 into the small end of the chimney and 

 cutting a slice from it just above the 

 chinniey's edge (leather may be used if 

 rubber is not available). This rubber 

 disk (r) is to be impaled on an um- 

 brella wire, as shown in Fig. 107 (the 

 larger of its surfaces is to be upper- 

 most), and firmly supported on one 

 side by a wooden disk {w) . The end 

 ^ of a spool is excellent for this purpose ; 

 it should be trimmed down to such 

 dimensions that it will slip easily into 

 the chimney. Below the wooden disk 

 we place a little sealing-wax to keep it 

 from slipping off ; above the rubber 

 disk we also place sealing-wax, as 

 shown in the figure. All the joints 

 must be made air-tight with sealing-wax ; this is 

 facilitated by filling the core of the spool with seal- 

 ing-wax, sharpening the end of the wire, heating it 

 and forcing it through the center of the rubber disk, 

 and then through the core of the spool. We must not 

 be discouraged by a few failures in constructing the 

 apparatus, since, after a little practice, we shall be able 



107. Details of piston 

 of air-pump. 



