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PEOrESSOE  TYNDALL  ON  SOME  PHYSICAL  PEOPEETIES  OF  ICE. 
I^ote  from  Mr.  Faeaday. 
My  deae  Tyndall, 
Have  the  following  remarks,  made  in  reference  to  the  irregular  fusibility  of  ice,  to 
which  you  drew  my  attention,  any  interest  to  you,  or  by  an  occasional  bearing  on  such 
cases,  any  value  in  themselves  % Deal  with  them  as  you  like. 
Imagine  a portion  of  the  water  of  a lake  about  to  freeze,  the 
surface  S being  in  contact  with  an  atmosphere  considerably  below 
32°,  the  previous  action  of  which  has  been  to  lower  the  tempe-  350 
rature  of  the  whole  mass  of  water,  so  that  the  portion  below  the  36° 
line  M is  at  40°,  or  the  maximum  density,  and  the  part  above  at 
progressive  temperatures  from  40°  upwards  to  32° ; each  stratum  39° 
keeping  its  place  by  its  relative  specific  gravity  to  the  rest,  and  40° 
having  therefore,  in  that  respect,  no  tendency  to  form  currents 
either  upwards  or  downwards.  Now  generally,  if  the  surface 
became  ice,  the  water  below  would  go  on  freezing  by  the  cold  conducted  downwards 
through  the  ice ; but  the  successive  series  of  temperatures  from  32°  to  40°  would  always 
exist  in  a layer  of  water  contained  between  the  ice  and  the  dense  water  at  40°  below  M. 
If  the  water  were  jpure,  no  action  of  the  cold  would  tend  to  change  the  places  of  the 
particles  of  the  water  or  cause  currents ; because  the  lower  the  cold  descended,  the  more 
firmly  would  any  given  particle  tend  to  retain  its  place  above  those  beneath  it : a par- 
ticle at  e,  for  instance,  at  36°  Fare.,  would,  when  the  cold  had  frozen  what  was  above 
it,  be  cooled  sooner  and  more  than  any  of  the  particles  beneath,  and  so  always  retain  its 
upper  place  as  respects  them. 
But  now,  suppose  the  water  to  contain  a trace  of  sahne  matters  in  solution.  As  the 
water  at  32°  froze,  either  at  the  surface  or  against  the  bottom  of  the  preHously-formed 
ice,  these  salts  would  be  expelled ; for  the  ice  fii’st  formed  (and  that  always  formed,  if 
the  proper  care  be  taken  to  displace  the  excluded  salts)  is  perfectly  free  from  them,  and 
PUEE.  The  salts  so  excluded  would  pass  into  the  layer  of  water  beneath,  and  there  pro- 
duce two  effects : they  would  make  that  layer  of  greater  specific  graHty  than  before, 
and  so  give  it  a tendency  to  sink  into  the  warmer  under  layer ; but  they  would  also 
make  it  require  a lower  temperature  than  32°  for  congelation ; this  it  would  acqufre 
from  the  cold  ice  above,  and  by  that  it  would  become  lighter  and  float,  tendmg  to 
remain  uppermost ; for  it  has  already  been  shown  that  the  diminution  of  temperature 
below  32°  in  sea  water  and  solution  of  salts,  is  accompanied  by  the  same  enlargement  of 
bulk  as  between  32°  and  40°  with  pure  water.  The  stratum  of  water,  therefore,  below 
the  ice,  would  not  of  necessity  sink  because  it  contained  a little  more  salt  than  the  stra- 
tum immediately  below  it ; and  certainly  would  not  if  the  increase  of  gravity  conferred 
by  the  salts  was  less  than  the  decrease  by  lowering  of  temperature.  An  approximation 
of  the  strata  between  the  freezing  place  and  the  layer  at  40°  would  occur,  i.  e.  the 
distance  between  these  temperatures  would  be  less,  but  the  water  particles  would  keep 
their  respective  places. 
s 
