Surface  Elasticity  of  Saponine  Solutions.  327 
will  have  very  little  effect.  On  the  other  hand,  the  conden- 
sation of  moisture  on  the  surface  will  have  a  marked  effect 
in  diminishing  the  surface  elasticity,  the  magnitude  of  this 
effect  depending  on  the  relation  between  the  rates  of  forma- 
tion of  the  water-film  by  condensation,  and  of  the  separation 
of  the  saponine  in  the  new  surface-layer  thus  produced. 
Hence  the  effect  of  alternating  periods  of  evaporation  and 
condensation,  even  though  the  amount  of  evaporation  is  much 
greater  than  the  amount  of  condensation,  will  be  to  prevent 
the  surface  from  ever  attaining  the  degree  of  elasticity 
attained  when  the  surface  is  kept  "dry." 
This  effect  was  graphically  illustrated  by  an  experiment 
carried  out  during  the  cold  weather  in  the  early  part  of 
January.  The  above  apparatus  was  used,  but  no  drying 
agent  was  employed.  The  nights  were  very  cold  and  the 
room  was  strongly  heated  during  the  day.  In  the  morning 
the  inside  of  the  bell-jar  was  covered  with  drops  of  water, 
which  had  distilled  over  from  the  solution.  Daring  the  day 
the  walls  gradually  dried,  the  moisture  distilling  back  to  the 
now  colder  interior  portions  of  the  bell-jar,  i.e.  to  the  surface. 
Corresponding  to  this  to-and-fro  distillation  of  moisture  were 
observed  fluctuations  in  the  elasticity.  A  1  per  cent,  solution 
was  used,  and  the  value  of  the  coefficient  of  rigidity  fluctuated 
from  about  100  dynes  per  cm.  in  the  morning  to  about 
350  dynes  per  cm.  in  the  evening.  The  experiment  was 
conducted  for  several  days,  and  there  was  no  progressive 
increase  such  as  occurs  when  continuous  evaporation  is 
promoted  (see  Table  II.). 
In  the  following  table  are  given  the  values  of  the  coefficient 
of  rigidity  attained  by  different  solutions  after  standing  about 
21  hours  (a)  exposed  to  the  air  of  the  room,  (b)  in  the 
apparatus. 
Table  III. 
Concentration  :  1  part  in 
a. 
b. 
10,000 
1,000,000 
10,000,000 
401 
92 
0 
535 
390 
81 
It  will  be  seen  that  the  more  dilute  the  solution  the  greater 
the  effect,o£  continuous  and  rapid  evaporation.  It  is  probable 
that  in  the  case  of  verv  dilute  solutions   the  effect  is  more 
