PEOFESSOE  TYNDALL  ON  SOME  PHYSICAL  PEOPEETIES  OF  ICE. 
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57.  The  surfaces  were  developed  with  great  facility  where  they  corresponded  to  the 
surfaces  of  freezing.  Wherever  the  liquid  disks  before  described  were  observed,  the 
surfaces  were  always  easily  developed  in  the  planes  of  the  disks.  By  care  I succeeded, 
in  some  cases,  in  producing  similar  effects  in  surfaces  at  right  angles  to  the  planes  of 
the  disks,  but  this  was  very  difficult  and  uncertain. 
I think  the  following  new  facts  have  been  established  in  the  foregoing  paper : — 
1.  The  interior  of  a mass  of  ice  may  be  melted  by  radiant  heat  which  has  passed 
through  exterior  portions  of  the  substance  without  melting  them. 
2.  In  the  dissolution  of  the  ice  thus  effected,  the  substance  yields  by  the  formation  of 
liquid  spaces,  each  possessing  the  shape  of  a flower  with  six  petals,  a small  vacuum  being 
formed  at  the  centre  of  each  flower. 
3.  These  flowers  are  generally  formed  in  planes  parallel  to  the  surface  of  freezing, 
but  some  of  the  specimens  of  ice  examined  were  dmded  into  prismatic  segments  by  sur- 
faces of  discontinuity.  Each  distinct  segment  had  its  own  system  of  flowers  arranged  in 
parallel  planes,  but  the  parallelism  did  not  extend  to  the  flowers  of  two  distinct  seg- 
ments. This  subject  requires  further  investigation. 
4.  The  appearance  presented  by  masses  of  lake  ice  composed  of  these  segments, 
resembles  that  of  certain  specimens  of  glacier  ice  in  which  the  air-  and  water-cavities  are 
flattened  in  different  planes.  Hence  the  explanation  of  the  latter,  which  refers  them  to 
actions  peculiar  to  the  glacier,  must  be  received  with  caution. 
5.  The  explanation  hitherto  given  of  the  water  associated  with  the  air-bubbles  in  gla- 
cier ice  is  untenable.  In  this  paper  the  phenomenon  is  explained  in  accordance  with 
the  dynamical  theory  of  heat.  It  is  sought  to  be  sho^vn  that,  owing  to  the  mutual 
action  of  the  ice-molecules,  a quantity  of  heat  which  has  been  conducted  through  the 
substance  without  prejudice  to  its  solidity  may  liberate  the  molecules  which  bound  an 
internal  cavity,  and  thus  produce  water-cells  in  association  with  the  bubbles  of  air. 
6.  The  converse  of  this  takes  place  where  two  moist  surfaces  of  ice  at  32°  Fahe.  are 
brought  into  contact.  Superficial  portions  are  thus  \firtually  rendered  central ; lique- 
faction is  checked,  the  film  of  moisture  on  the  surfaces  in  contact  congeals,  and  the 
pieces  of  ice  freeze  together.  To  this  process  the  term  regelation  has  been  applied. 
7.  By  the  application  of  pressure  parallel  surfaces  of  discontinuity  are  formed  in  lake 
and  river  ice  perpendicular  to  the  directions  in  which  the  pressure  is  exerted ; thus 
giving  the  substance  the  appearance  of  selenite,  in  which  the  planes  of  cleavage  are  not 
in  optical  contact.  The  discontinuity  consists  in  the  liquefaction  of  the  ice  in  these 
planes  by  the  pressure.  Such  smffaces  are  formed  with  great  facility  parallel  to  the 
planes  in  which  the  liquid  flowers  are  formed  by  radiant  heat,  while  it  is  very  difficult 
to  produce  them  perpendicular  to  these  planes.  Thus,  whether  we  apply  heat  or 
pressure,  lake  ice  melts  with  peculiar  facility  in  certain  directions. 
2 H 2 
