214 
PEOFESSOE  TYNDALL  ON  SOME  PHYSICAL  PEOPEETIES  OF  ICE. 
circular  spots  of  oily  scum  which  float  on  the  surface  of  mutton  broth,  and  in  the  pieces 
of  ice  flrst  examined  they  always  lay  in  the  planes  of  freezing. 
14.  As  time  progressed  this  internal  disintegration  of  the  ice  appeared  to  become 
more  pronounced,  so  that  some  pieces  of  Norway  ice  examined  in  the  middle  of  Novem- 
ber, appeared  to  be  reduced  to  a congeries  of  water-ceUs  entangled  in  a skeleton  of  ice. 
The  effect  of  this  was  rendered  manifest  to  the  hand  on  sawing  a block  of  this  ice,  by 
the  facility  with  which  the  saw  went  through  it. 
15.  There  seems  to  be  no  such  thing  as  absolute  homogeneity  in  nature.  Change 
commences  at  distinct  centres  instead  of  being  uniformly  and  continuously  distributed, 
and  in  the  most  apparently  homogeneous  substance  we  should  discover  defects,  if  our 
means  of  observation  were  fine  enough.  The  above  observations  show  that  some  por- 
tions of  a mass  of  ice  melt  more  readily  than  others.  The  melting  temperature  of  the 
substance  is  set  down  at  32°  Fahe.,  but  the  absence  of  perfect  homogeneity,  whether 
from  difference  of  crystalline  texture  or  some  other  cause*,  makes  the  melting  tempe- 
rature oscillate  to  a slight  extent  on  both  sides  of  the  ordinary  standard.  Let  this  limit 
expressed  in  parts  of  a degree  be  t.  Some  parts  of  a block  of  ice  will  melt  at  a tempe- 
rature of  32  — t,  while  others  require  a temperature  of  32 the  consequence  udll  be 
that  such  a block  raised  to  the  temperature  of  32°  will  have  some  of  its  parts  liquid, 
and  others  solid. 
16.  When  a mass  exhibiting  the  water-disks  was  examined  by  a concentrated  sun-beam, 
the  six  leaved  flowers  before  referred  to  were  always  formed  in  the  planes  of  the  disks. 
17.  In  all  my  earlier  experiments  I found  the  rule  to  hold  good,  that  both  disks  and 
flowers  were  developed  in  the  planes  of  freezing,  but  I was  subsequently  siu-prised  to 
find,  in  the  self-same  mass  of  ice,  the  disks  lying  in  different  planes.  On  examining 
such  pieces  I found  them  traversed  by  hazy  surfaces  of  discontinuity,  which  di-\ided  the 
apparently  continuous  mass  into  irregular  prismatic  segments.  When  examined  by 
allowing  the  red  light  of  a fire  to  cross  it,  such  ice  had  a beautiful  appearance.  The 
interior  walls  of  the  segments  were  thickly  covered  with  rich  liquid  disks;  in  some 
cases  the  vision  plunged  unimpeded  into  the  ice  to  a depth  of  several  inches,  while  in 
others  the  prismatic  segments  were  dotted  with  disks  to  their  very  centres.  Fig.  1 
Fig.  1. 
