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PEOFESSOE  TTIfDALL  ON  SOME  PHYSICAL  PEOPEETIES  OF  ICE. 
52.  To  ascertain  whether  the  rupture  of  optical  contact  which  these  experiments 
disclosed  was  due  to  the  intrusion  of  air  between  two  separated  surfaces  of  ice,  a 
cyhnder  of  ice,  2 inches  long  and  1 inch  wide,  was  placed  in  a copper  vessel  containing 
ice-cold  water.  The  ice-cylinder  projected  half  an  inch  above  the  surface  of  the  water. 
Placing  the  copper  vessel  on  a slab  of  wood,  and  a second  slab  of  wood  upon  the 
cylinder  of  ice,  the  whole  was  subjected  to  pressure.  When  the  hazy  surfaces  were 
well  developed  in  the  portion  of  the  ice  above  the  water,  the  cylinder  was  removed 
and  examined.  The  planes  of  rupture  extended  throughout  the  entire  length  of  the 
cylinder,  just  the  same  as  if  it  had  been  squeezed  in  free  ah’. 
Still  the  removal  of  the  cylinder  from  its  vessel  might  be  attended  Avith  the  intrusion 
of  air  into  the  fissures.  I therefore  placed  a cylinder  of  ice,  2 inches  long  and  1 inch  wide, 
in  a stout  vessel  of  glass,  which  was  filled  with  ice-cold  water.  Squeezing  the  whole,  as 
in  the  last  experiment,  the  surfaces  of  discontinuity  were  seen  forming  under  the  liquid 
quite  as  distinctly  as  in  air. 
53.  The  surfaces  are  due  to  compression,  and  not  to  any  tearing  asunder  of  the  mass 
by  tension,  and  they  are  best  developed  where  the  pressm’e,  within  the  limits  of  frac- 
ture, is  a maximum.  A cylindrical  piece  of 
ice,  one  of  whose  ends  was  not  parallel  to  the 
other,  was  placed  between  slabs  of  wood  and 
subjected  to  pressure.  Fig.  4 shows  the  dispo- 
sition of  the  experiment.  The  effect  upon  the 
ice-cylinder  was  that  shown  in  fig.  5,  the  sur- 
faces being  developed  along  that  side  which 
had  suffered  the  pressure. 
54.  Sometimes  the  surfaces  commence  at  the  centre  of  the  cylinder.  A dhn  small 
spot  is  first  observed,  which,  as  the  pressure  continues,  expands  until  it  sometimes 
embraces  the  entire  transverse  section  of  the  cylinder. 
55.  On  examining  these  surfaces  with  a pocket-lens,  they  appeared  to  me  to  be  com- 
posed of  very  minute  water  parcels,  hke  what  is  produced  upon  a smooth  cold  sm-face 
by  the  act  of  breathing.  Were  they  either  vacuous  plates,  or  plates  filled  with  air, 
their  aspect  would,  on  optical  grounds,  be  far  more  vivid  than  it  really  was. 
56.  A concave  mirror  was  so  disposed,  that  the  diffused  light  of  day  was  throum  full 
upon  the  cyhnder  while  under  pressure.  Observing  the  expanding  surfaces  through  a 
lens,  they  appeared  in  a state  of  intense  commotion;  this  was  probably  due  to  the 
molecular  tensions  of  the  httle  water  parcels.  This  motion  followed  closely  on  the  edge 
of  the  surface  as  it  advanced  through  the  solid  ice.  Once  or  tmce  I observed  the  hazy 
surfaces  pioneered  through  the  mass  by  dim  offshoots  apparently  liquid.  They  consti- 
tuted a kind  of  negative  crystallization,  having  the  exact  form  of  the  crystalline  spines 
and  spurs  produced  by  the  congelation  of  water  upon  a surface  of  glass.  I have  no 
doubt,  then,  that  these  surfaces  are  produced  by  the  liquefaction  of  the  solid  in  planes 
perpendicular  to  the  direction  of  pressure. 
Fig.  5. 
Fig.  4. 
