Torsional  Stress  in  Solutions  of  Gelatine. 
451 
point  is  free  to  recover  without  having  first  to  overcome 
friction.     In  tins  way  a  series  of  smooth  curves  was  obtained. 
Another  precaution  had  to  be  taken.  Preliminary  experi- 
ments sufficed  to  show  that  the  consistency  of  any  specimen 
of  jelly  depended  largely  on  the  temperature.  For  purposes 
of  comparison  between  solutions  of  different  concentrations, 
therefore,  it  was  necessary  to  maintain  them  throughout  the 
experiment  as  far  as  possible  at  equal  temperatures.  To 
effect  this  the  outer  cylinder  containing  the  jelly  was 
surrounded  by  a  water-jacket  which  was  itself  wrapped  round 
and  round  with  felt.  It  was  then  possible  to  keep  the 
temperature  of  the  jelly  fairly  constant  for  about  three  hours. 
The  following  table  shows  the  variation  of  temperature  in 
the  actual  experiments  : — 
Experiment. 
Percentage 
of  Gelatine. 
Maximum  &  Minimum 
Temperatures. 
Centigrade  degrees. 
Mean  Temperature 
for  two  hours. 
1 
34 
4-0 
45 
4-2 
4-4 
16-32-16-44 
16-23-16-59 
16-10-16-52 
16-15-16-63 
16-18-16-76 
.  16-38 
16-41 
16-31 
16-39 
16-47 
2 
3 
4 
5 
II.   Preparation  of  Solutions. 
The  consistency  of  any  specimen  of  jelly  does  not  solely 
depend  on  the  percentage  of  gelatine  present  and  the 
temperature.  It  depends  also  on  the  way  in  which  the 
solution  is  prepared  and  the  time  allowed  for  setting.  Thus, 
if  one  solution  of  gelatine  be  boiled  for  some  time  and  another 
of  equal  concentration  be  made  simply  by  dissolving  the 
gelatine  in  hot  water  without  boiling,  then,  even  if  the  times 
of  setting  are  the  same,  the  two  have  different  consistencies. 
A  gradual  hardening,  too,  occurs  in  any  jelly  even  after  the 
temperature  of  the  atmosphere  has  been  reached. 
In  order  to  compare  specimens  varying  only  in  con- 
centration, it  was  essential  to  ensure  that  all  the  other 
conditions  determining  the  consistency  remained  constant. 
The  temperature  was  kept  constant  in  the  way  before 
described.  All  the  solutions  were  prepared  by  simply  dis- 
solving the  gelatine  in  water  that  had  just  been  boiled  and 
making  up  to  the  required  concentration  by  the  addition  of 
hot  water.  The  liquid  (equal  volumes  in  each  case)  was  then 
poured  into  the  interspace  between  the  two  glass  cylinders  and 
allowed  to  set  for  a  Hxod  time — 2'2h  hours. 
