PEOFESSOE  TYNDALL  ON  SOME  PHYSICAL  PEOPEETIES  OF  ICE. 
225 
§ VI. 
50.  In  a very  interesting  paper  communicated  to  the  British  Association  during  its 
last  meeting,  Mr.  James  Thomson  has  explained  the  freezing  together  of  two  pieces  of 
ice  at  32°  in  the  following  manner : — “ The  two  pieces  of  ice,  on  being  pressed  together 
at  their  point  of  contact,  will  at  that  place,  in  mtue  of  the  pressure,  be  in  part  liquefied 
and  reduced  in  temperature,  and  the  cold  evolved  in  theh  liquefaction  will  cause  some 
of  the  hquid  film  intervening  between  the  two  masses  to  freeze.”  I am  far  from  denying 
the  operation  under  proper  circumstances  of  the  mra  causa  to  which  Mr.  Thomson 
refers,  but  I do  not  think  it  explains  the  facts.  For  freezing  takes  place  without  the 
intervention  of  any  pressure  by  which  Mr.  Thomson’s  efiect  could  sensibly  come  into 
play.  It  is  not  necessary  to  squeeze  the  pieces  of  ice  together ; one  bit  may  be  simply 
laid  upon  the  other  and  they  will  still  freeze.  Other  substances  besides  ice  are  also 
capable  of  being  frozen  to  the  ice.  If  a towel  be  folded  round  a piece  of  ice  at  32°, 
they  will  freeze  together.  Flannel  is  still  better.  A piece  of  flannel  wrapped  round  a 
piece  of  ice  freezes  to  it  sometimes  so  firmly  that  a strong  tearing  force  is  necessary  to 
separate  both.  Cotton  wool  and  hair  may  also  be  fi’ozen  to  ice  without  the  intervention 
of  any  pressure  which  could  render  Mr.  Thomson’s  cause  sensibly  active*. 
51.  But  there  is  a class  of  efiects  to  the  explanation  of  which  the  lowering  of  the  freezing- 
point  of  water  by  pressure  may,  I think,  be  properly  applied.  The  following  statement 
is  true  of  fifty  experiments  or  more  made  with  ice  from  various  quarters.  A cylinder  of 
ice  2 inches  high  and  an  inch  in  diameter,  was  placed  between  two  slabs  of  box-wood 
and  submitted  to  a gradually  increasing  pressure.  Looked  at  perpendicular  to  the  axis, 
cloudy  lines  were  seen  drawing  themselves  across  the  cylinder ; and  when  the  latter  was 
looked  at  obliquely,  these  lines  were  found  to  be  the  sections  of  dim  hazy  surfaces  which 
traversed  the  cylinder,  and  gave  it  an  appearance  closely  resembling  that  of  a crystal  of 
gypsum  whose  planes  of  cleavage  had  been  forced  out  of  optical  contact  by  some  external 
force. 
Fig.  2 represents  the  cyhnder  looked  at  perpendicular  to  its  axis,  and  fig.  3 the  same 
cylinder  where  looked  at  obliquely. 
Fig.  2.  Fig.  3. 
tr  ^ 
— 
— — - ^ 
— ~ ^ M i 
* It  miglit  perhaps  be  stated  generally,  that  those  substances  which  cause  the  precipitation  of  water  from 
the  gaseous  to  the  liquid  state,  are  als^  most  influential  in  converting  the  liquid  into  the  solid.  “ Hemp, 
wool,  hair,”  are  also  the  substances  on  which  the  so-called  ground-ice  is  formed  most  readily.  See  Aeago’s 
Eeport,  Annuaire  pour  Van  1833. 
MDCCCLVIII.  2 H 
