Surface  Elasticity  of  Saponine  Solutions. 
325 
on  three  levelling- screws.  The  bell-jar  was  luted  to  the  base 
by  means  o£  mercury.  In  the  case  of  long-exposed  surfaces, 
oscillations  could  be  set  up  by  merely  pressing  in  a  horizontal 
direction  against  the  end  of  the  arm  E,  which  was  slightly 
23  l..^\ 
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flexible,  without  visibly  moving  it.  Owing  to  the  fragility  of 
the  surface  this  method  was  preferable  to  actually  rotating 
the  platform  through  a  comparatively  large  angle.  The 
motion  was  observed  by  focussing  a  short-focus  telescope  on 
the  reflexion  of  the  filament  of  an  incandescent  electric  lamp. 
By  this  means  several  well-defined  lines  were  obtained,  in 
spite  of  the  thickness  and  curvature  of  the  glass;  and  by 
observing  the  motion  of  one  of  these  relative  to  the  dust- 
particles  in  the  eyepiece  of  the  telescope,  the  end-points  of 
the  oscillations  could  be  quite  accurately  noted. 
The  condensation  of  moisture  on  the  surface  was  prevented 
by  placing  a  number  of  shallow  vessels  containing  calcium 
chloride  on  the  base.  A  deeper  vessel  containing  a  little  of 
the  drying  agent  was  suspended  by  means  of  a  loop  of  wire 
from  the  stopper. 
