PEOFESSOE  TYNDALL  ON  SOME  PHYSICAL  PEOPEETIES  OF  ICE. 
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37.  A second  piece,  similarly  frozen  and  wrapped  up  in  flannel,  showed  the  same 
deportment.  In  an  hour  and  a half  the  frozen  water  surrounding  the  air-bubbles  was 
restored  to  its  liquid  condition.  Hence  no  doubt  can  remain  as  to  the  possibility  of 
effectmg  liquefaction  in  the  interior  of  a mass  of  ice,  by  heat  which  has  passed  by 
conduction  through  the  substance  without  melting  it. 
38.  I have  ah-eady  referred  to  the  formation  of  the  liquid  cavities  observed  by 
M.  Agassiz,  when  glacier  ice  was  exposed  to  the  sun.  The  same  effect  may  be  pro- 
duced by  exposure  to  a glowing  coal  fire.  On  the  21st  and  22nd  of  November  I thus 
exposed  plates  of  clear  Wenham  Lake  ice,  which  contained  some  scattered  air-bubbles. 
At  first  the  bubbles  were  sharply  rounded,  and  without  any  trace  of  water.  Soon,  how- 
ever, those  near  the  surface,  on  which  the  radiant  heat  fell,  appeared  encu’cled  by  a 
liquid  ring,  which  expanded  and  finally  became  crimped  at  its  border,  as  shown 
in  the  adjacent  figure.  The  crimping  became  more  pronounced  as  the  action  was 
permitted  to  continue*. 
A second  plate,  crowded  with  bubbles,  was  held  as  near  to  the  fire  as  the  hand  could 
bear.  On  withdrawing  it,  and  examining  it  through  a pocket  lens,  the  appearance  was 
perfectly  beautiful.  In  many  cases  the  bubbles  appeared  to  be  surrounded  by  a series 
of  concentric  rings,  the  outer  ring  surrounding  all  the  others  like  a crimped  frill. 
39.  I could  not  obtain  these  effects  by  placing  the  ice  in  contact  with  a plate  of  metal 
obscurely  heated f,  nor  by  the  radiation  from  an  obscure  source.  Indeed  ice,  as  before 
remarked,  is  impervious  to  radiant  heat  from  such  a source  J.  The  rays  from  a common 
fire  also  are  wholly  absorbed  near  the  surface  upon  which  they  strike,  and  hence  the 
described  internal  liquefaction  was  confined  to  a thin  layer  close  to  this  surface. 
40.  But  not  only  does  liquefaction  occur  in  connexion  with  the  bubbles,  but  the 
“ flowers,”  already  described  as  produced  by  the  solar  beams,  start  by  hundreds  into 
existence,  when  a slab  of  transparent  ice  is  placed  before  a glowing  coal  fire.  They, 
however,  are  also  confined  to  a thin  stratum  of  the  substance  close  to  the  surface  of 
incidence.  In  the  experiments  made  in  this  way,  the  central  stars  of  the  flowers  were 
often  bounded  by  sinuous  lines  of  great  beauty. 
41.  The  foregoing  considerations  show  that  liquefaction  takes  place  at  the  surface  of 
a mass  of  ice  at  a lower  temperature  than  that  required  to  liquefy  the  interior  of  the 
solid.  At  the  surface  the  temperature  32°  produces  a vibration,  to  produce  which, 
within  the  ice,  would  necessitate  a temperature  of  the  increment  x being  the 
* The  blebs  observed  in  glacier  ice  also  exhibit  this  form  : see  tig.  8,  plate  6,  of  the  Atlas  to  the  ‘ Systeme 
Glaciaire.’  In  fig.  13  we  have  also  a close  resemblance  of  the  flower-shaped  figures  produced  by  radiant 
heat  in  lake  ice. 
t To  develope  water-cavities  within  ice  a considerable  time  is  necessary ; more  time  indeed  than  was  suffi- 
cient to  melt  the  entire  pieces  of  ice  made  use  of  in  these  contact  experiments. 
X Hence  the  soundness  of  the  ice  under  the  moraines ; the  sun’s  rays  are  converted  into  obscure  heat  by 
the  overlying  debris ; this  only  aflfects  a layer  of  infinitesimal  depth,  and  cannot  produce  the  disintegration 
of  the  deeper  ice,  as  the  direct  sunbeams  can. 
