448  Mr.  A.  0.  Rankine  on  the  Decay  of 
and  at  the  same  time  it  is  possible  with  high  concentrations 
to  make  specimens  which  are  perfectly  elastic  for  fairly  large 
stresses.  Intermediate  concentrations  give  elastic  and  viscous 
properties  in  varied  proportions.  Gelatine  was  therefore 
chosen  for  the  investigation  of  the  question  proposed  above. 
The  consistency  of  solutions  of  gelatine  in  water  which  have 
been  allowed  to  congeal  is  very  varied  in  character.  All  of 
these,  however,  seem  to  possess,  to  a  greater  or  less  degree, 
elastic  properties.  It  also  appears  that  any  particular  spe- 
cimen has  a  definite  elastic  limit  provided  that  none  of  the 
conditions  which  determine  its  consistency  vary.  It  may  be 
subjected  to  a  stress  of  less  than  a  certain  limiting  value,  and 
the  resulting  strain  will  be  constant  with  regard  to  time  and 
it  will  recover  on  the  removal  of  the  stress  to  its  original 
state.  If,  however,  this  limiting  stress  is  exceeded,  the  strain 
is  no  longer  constant  but  increases  with  time.  The  removal 
of  the  stress  does  not  now  produce  complete  recovery,  although 
it  should  be  noted  that  if  the  stress  is  only  applied  momen- 
tarily the  recovery  is  very  nearly  complete. 
Now  suppose  a  specimen  of  jelly  stressed  beyond  its  elastic 
limit.  There  will  result  an  immediate  strain  followed  by  a 
gradual  increase  with  time.  This  increase,  however,  can  be 
prevented  by  the  gradual  removal  of  the  stress,  and  in  order 
to  keep  the  strain  constant  this  removal  must  be  continued 
until  the  remaining  stress  is  equal  to  the  elastic  limit  of 
stress  for  the  particular  specimen. 
It  was  to  investigate  this  change  in  the  special  case  of  the 
rate  of  decrease  of  couple  in  solutions  of  gelatine  twisted 
through  a  constant  amount,  that  the  following  experiments 
were  carried  out. 
I.  Method  of  Experiment. 
The  method  used  has  been  to  apply  equal  torsional  stresses 
to  gelatine  solutions  of  different  concentrations,  to  maintain 
the  strain  always  equal  to  the  initial  immediate  strain,  and  to 
record  the  rate  of  falling  off  of  the  stress.  It  was  arranged 
that  the  removal  of  the  stress  should  be  effected  automatically 
by  a  method  very  similar  to  that  used  by  Professor  Trouton 
and  myself  in  our  experiments  on  lead  wires  (Phil.  Mag. 
October  1904). 
The  interspace  between  two  concentric  glass  cylinders  is 
filled  with  a  prepared  solution  of  gelatine  which  is  allowed  to 
set.  A  torsional  stress  can  then  be  produced  in  the  jelly  by 
twisting  the  inner  cylinder  and  retaining  the  outer  one  fixed. 
This  inner  cylinder  forms  the  lower  part  A  B  of  figure  1,  and 
is  rigidly  fixed  to  the  brass  piece  C  D  which  is  itself  supported 
