Surface  Elasticity  of  Saponine  Solutions. 
323 
influence  on  the  initial  elasticity.  Some  of  the  earlier  experi- 
ments seemed  to  indicate  that  the  initial  elasticity  was 
slightly  greater  i£  the  liquid  was  poured  out  so  as  to  splash 
and  form  bubbles  on  the  surface,  than  if  it  was  poured  out 
with  the  minimum  of  mechanical  disturbance,  but  this  result 
was  not  satisfactorily  verified  by  subsequent  experiments. 
No  difference  in  the  initial  elasticity  between  the  con- 
centration limits  1  in  100  and  1  in  10,000  could  be  detected. 
Between  the  concentrations  1  in  10,000  and  1  in  100,000  a 
rapid  change  occurs.  Experiments  carried  out  with  solutions 
of  intermediate  strengths  gave  the  following  results  : — 
Table  I. 
Concentration:  1  part  in 
T  (initial). 
15,000 
16,000 
17,000 
20,000 
2*5  to  3  sec. 
3  sec  to  go  . 
Motion  oscillatory  of  the 
first   type  with  exces- 
J       sive  damping. 
It  does  not  follow  that  any  specially  marked  change  occurs 
between  the  concentrations  1  in  15,000  and  1  in  17,000. 
The  point  at  which  the  effect  of  the  surface  changes  from 
elastic  to  viscous  depends  upon  the  dimensions  of  the  appa- 
ratus. Thus,  by  using  a  vessel  with  a  diameter  of  8  cms., 
all  the  above  solutions  show  an  initial  elastic  effect.  By 
using  a  disk  with  a  very  small  moment  of  inertia  and  having 
the  annular  space  between  the  disk  and  vessel  narrow,  an 
initial  elastic  effect  can  be  obtained  with  solutions  much 
more  dilute  than  the  above.  There  is  a  limit,  however,  in 
practice  to  the  effectiveness  of  such  arrangements,  due  to  the 
fact  that  a  certain  amount  of  liquid  moves  with  the  disk  and 
surface,  so  that  however  light  the  suspended  system  there  is 
a  lower  limit  to  the  inertia  of  the  total  moving  system.  In 
the  case  of  the  1  in  100,000  solution,  it  is  impossible  to 
obtain  an  initial  elastic  effect  even  with  the  most  favourable 
arrangements.  The  surface  has,  however,  a  marked  initial 
viscosity. 
The  excessive  rapidity  of  formation  of  the  elastic  surface 
in  the  case  of  the  stronger  solutions  is  shown  by  the  fact  that 
it  is  impossible  to  destroy,  or  even  temporarily  impair,  the 
elasticity  by  mechanical  means  such  as  stirring  the  liquid  with 
