321  Mr.  S.  A.  Shorter  on  the 
a  glass  rod.     In  fact,  the  only  effect  o£  such  an  operation  is 
to  set  up  oscillations  of  the  second  type. 
In  the  case  of  the  1  in  10'5  and  1  in  10°  solutions,  there  was 
a  rapid  increase  of  superficial  viscosity  on  standing,  and  the 
surface  gradually  developed  an  elastic  effect  as  previously 
described.  The  1  in  107  solution  showed  no  trace  of  elasticity, 
or  even  of  marked  viscosity,  after  24  hours.  The  properties 
of  elastic  surfaces  formed  after  prolonged  standing  will  be 
next  investigated. 
The  Elasticity  of  Long -exposed  Surfaces. 
After  prolonged  exposure  of  the  surface  the  elastic  forces 
attain  a  high  degree  of  permanence.  In  many  cases  the  disk 
may  be  made  to  oscillate  for  about  20  seconds.  At  this 
stage  the  surface  cannot  be  described  as  viscous.  For  small 
motions  which  do  not  shear  it  beyond  its  elastic  limit,  the 
surface  behaves  as  an  elastic  rather  than  a  viscous  body.  It 
is  only  under  excessive  shear  that  the  surface  shows  a  viscous 
effect. 
On  measuring  the  surface  elasticity  at  intervals  extending 
over  several  days,  it  was  found  that  after  the  initial  period 
of  rapid  increase  there  were  considerable  fluctuations  during 
the  course  of  the  clay.  The  elasticity  was  generally  greatest 
in  the  morning  and  decreased  during  the  day.  These 
fluctuations  were  most  pronounced  if  the  temperature  of  the 
room  was  much  higher  during  the  day  than  during  the  night. 
Further  investigation  of  this  point  showed  that  these  fluctua- 
tions were  not  primarily  due  to  temperature  changes.  The 
decreases  which  occurred  during  the  day  were  found  to  be 
due  to  the  condensation  of  water-vapour  from  the  air  of  the 
room  on  the  surface.  Thus  the  effect  was  very  marked  if 
after  a  cold  night  the  room  was  warmed  and  occupied  by  a 
number  of  persons  so  that  the  air  became  saturated  with 
moisture  at  a  temperature  much  above  that  of  the  solution. 
In  order  to  eliminate  this  effect  a  special  form  of  apparatus 
was  designed.  This  is  shown  in  the  accompanying  diagram 
(fig.  2).  A  is  a  bell-jar  35  cms.  high  and  18  cms.  in  diameter. 
From  a  short  piece  of  brass  rod  passing  through  the  centre  of 
the  stopper  B  was  hung,  by  means  of  a  thin  steel  wire,  the 
rod  carrying  the  disk  and  mirror.  Cis  a  wooden  base  with  a 
circular  groove  into  which  the  mouth  of  the  bell-jar  could  be 
fitted.  A  steel  screw  was  fixed  through  the  centre  of  the 
base.  To  the  upper  end  of  this  was  fixed  a  circular  piece  of 
wood,  D,  which  served  as  a  platform  for  the  vessel.  To  the 
lower  end  was  soldered  a  horizontal  radial  arm,  E,  which 
projected  slightly  from  under  the  base  which  was  supported 
