PEOrESSOE  TYNDALL  ON  SOME  PHYSICAL  PEOPEETIES  OF  ICE. 
229 
When  water  freezes,  it  does  not  appear  that  this  process  is  continuous,  for  many  of 
the  characters  of  the  ice  seem  to  show  that  it  is  intermittent ; e.  either  a film  of  ice  is 
formed,  and  then  the  process  stops  until  the  heat  evolved  by  solidification  has  been  con- 
ducted away  upwards,  and  the  next  stratum  of  water  has  been  sufficiently  cooled  to  freeze 
in  turn ; or  else  the  freezing  being,  so  to  speak,  continuous,  still  is  not  continued  at  the 
same  constant  rate,  but,  as  it  were,  by  intermittent  pulsations.  Now  it  may  well  be, 
when  a layer  next  the  previously-formed  ice,  and  containing  an  undue  proportion  of 
salts,  has  been  cooled  down  to  its  required  temperature  for  freezing  (which  would  be 
below  32°),  that  on  freezing,  the  congelation  will  pervade  at  once  a certain  thickness  of 
the  water,  excluding  the  salts  from  the  larger  portion  of  ice  formed,  but  including  them 
as  a weak  solution  within  its  interstices.  The  next  increment  of  cold  conducted  from 
the  ice  above  would  freeze  up  these  salts  in  the  ice  containing  them,  at  the  same  time 
that  a layer  of  pm’e  ice  was  formed  beneath  it.  Thus  a layer  of  ice  fusible  at  a lower 
temperature  than  the  ice  either  above  or  below  it  might  be  produced ; and  by  a repe- 
tition of  the  process  many  such  layers  might  be  formed. 
It  does  not  follow  necessarily  that  the  layers  would  be  perfectly  exact  in  their  dispo- 
sition. Very  slight  ciroumstances  tending  to  distm’b  the  regularity  of  the  water-mole- 
cules would  be  sufficient,  probably,  to  disturb  the  layers  more  or  less.  Ice  contains  no 
air,  and  the  exclusion  of  a minute  bubble  of  air  from  the  water  in  the  act  of  freezing 
might  disturb  the  direction  and  progress  of  the  congelation,  and  cause  accumulation  of 
the  extra  saline  hquid  in  one  spot  rather  than  another : so  might  the  tendency  to  the 
formation  of  httle  currents,  either  arising  from  the  separation  of  the  saline  water  from 
the  forming  ice,  or  from  the  elevation  of  temperature  in  different  degrees  at  those  places 
where  the  congelation  was  going  on  at  different  rates. 
The  effect  would  not  depend  upon  the  quantity  of  salts  contained  in  the  freezing 
water,  though  its  degree  would.  The  proportion  of  salts  necessary  to  be  added  to  pure 
water  to  lower  its  freezing-point  1°  Fahe.  may  be  very  sensible  to  chemical  tests,  but  the 
proportion  requh’ed  to  make  the  difference  TWoolli  of  ^ degree  would  be  far  less : 
and  if  we  suppose  that  only  -^th  of  a piece  of  ice  is  brought  into  the  condition  of  melt- 
ing before  the  rest  of  the  mass,  and  that  the  salts  in  that  proportion  were  originally  in 
the  whole  of  the  water,  then  its  quantity  there  may  he  so  small  as  to  escape  detection 
except  by  very  careful  analysis.  However,  it  would  be  desirable  to  examine  the  water 
chemically  which  is  produced  by  ice  distinguished  by  having  in  its  interior  much,  that 
liquefies  before  the  rest. 
It  is  easy  to  make  ice  perfectly  free  from  air,  and,  as  I believe,  from  salts,  by  a pro- 
cess I formerly  described.  It  would  be  interesting  to  see  if  such  ice  had  within  it  por- 
tions melting  at  a lower  temperature  than  the  general  mass.  I think  it  ought  not. 
Ever  truly  yours, 
M.  Faeaday. 
Royal  Institution^ 
^th  December,  1857. 
