PEOPESSOE  TYNDALL  ON  SOME  PHYSICAL  PEOPEETIES  OF  ICE. 
215 
represents  one  of  a number  of  cases  of  this  kind  which  I had  an  opportunity  of  exami- 
ning. The  network  of  lines  are  the  intersections  of  the  bounding  surfaces  of  the  segments 
with  the  surface  of  the  slab  of  ice : the  short  lines  in  each  segment  represent  the 
sections  of  the  disks ; and  it  will  be  observed  that  while  in  each  single  segment  their 
directions  are  ahke,  in  no  two  segments  is  this  the  case.  The  flat  rings  denote  that  the 
disks,  at  these  places,  were  parallel  to  the  surface  of  the  ice  slab,  while  in  all  the  other 
cases  they  were  perpendicular  to  it. 
18.  I placed  this  slab  on  a table  which  could  be  caused  to  rotate,  and  bringing  it 
into  the  path  of  a concentrated  sunbeam,  caused  the  beam  to  travel  all  round  it.  The 
little  flowers  started  into  existence  with  extreme  suddenness  and  beauty.  In  all  cases 
the  planes  of  the  flowers  were  parallel  to  the  planes  of  the  disks. 
19.  Hence  the  conclusion  that  the  flowers  are  always  parallel  to  the  surface  of  free- 
zing, must  not  go  without  qualiflcation.  There  is  no  doubt  that  each  of  the  fragments 
above  referred  to,  possesses  an  optic  axis  perpendicular  to  the  planes  of  the  flowers 
developed  in  it ; the  directions  of  these  axes  being  therefore  as  various  as  the  planes 
aforesaid.  How  is  this  result  brought  about '?  Has  the  molecular  structure  of  the  ice 
been  always  what  the  last  experiments  show  it  to  be  1 Sir  David  Beewster’s  observa- 
tions are  in  opposition  to  this  idea.  Or  can  it  be  that  the  ice  has  changed,  by  a rear- 
rangement of  the  constituent  particles  of  each  segment,  this  arrangement  producing  the 
surfaces  of  discontinuity  by  which  the  segments  are  bounded  ] At  present  we  are  con- 
flned  to  mere  conjectures,  but  I hope  the  coming  winter  will  enable  me  to  investigate 
this  highly  interesting  question. 
§III. 
20.  What  has  been  already  said  will  prepare  us  for  the  consideration  of  an  associated 
class  of  phenomena  of  great  physical  interest.  The  larger  masses  of  ice  which  I 
examined  exhibited  layers  in  which  bubbles  of  air  were  collected  in  unusual  quantity, 
marking,  no  doubt,  the  limits  of  successive  acts  of  freezing.  These  bubbles  were  usually 
elongated.  Between  two  such  beds  of  bubbles  a clear  stratum  of  ice  intervened ; and  a 
clear  surface  layer,  which,  from  its  appearance,  seemed  to  have  suffered  more  from 
external  influences  than  the  rest  of  the  ice,  was  associated  with  each  block.  In  this 
superficial  portion  I observed  detached  air-bubbles  irregularly  distributed,  and  associated 
with  each  vesicle  of  air,  a bleb  of  water  which  had  the  appearance  of  a drop  of  clear  oil 
within  the  solid.  The  adjacent  figm’e  will  give  a notion  of  these  com- 
posite cavities : the  unshaded  circle  represents  the  air-bubble,  and  the 
shaded  space  adjacent,  the  water. 
21.  When  the  quantity  of  water  was  sufficiently  large,  which  was 
usually  the  case,  on  tmTiing  the  ice  round,  the  bubble  shifted  its  posi- 
tion, rising  always  at  the  top  of  the  bleb  of  water.  Sometimes,  however,  the  cell  was 
very  flat,  and  the  air  was  then  quite  surrounded  by  the  liquid.  These  composite  cells 
often  occurred  in  pellucid  ice,  which  showed  inwardly  no  other  sign  of  disintegration. 
This  is  manifestly  the  same  phenomenon  as  that  which  struck  M.  Agassiz  so  forcibly 
