220  PEOFESSOE  TYXDALL  ON  SOME  PHYSICAL  PEOPEETIES  OF  ICE. 
day,  to  a motion  of  its  particles.  When  this  motion  reaches  such  an  intensity  as  to 
liberate  sufficiently  the  particles  of  a solid  from  their  mutual  attractions,  the  body  passes 
into  the  liquid  condition.  Now  as  regards  the  amount  of  motion  necessary  to  produce 
this  liberty  of  liquidity,  the  particles  at  the  surface  of  a mass  of  ice  must  be  very  differ- 
ently circumstanced  from  those  in  the  interior,  which  are  influenced  and  controlled  on 
every  side  by  other  particles.  But  if  we  suppose  a cavity  to  exist  nithin  the  mass,  the 
particles  bounding  that  cavity  will  be  in  a state  resembling  that  of  the  particles  at  the 
surface ; and  by  the  removal  of  all  opposing  action  on  one  side,  the  molecrrles  may  be 
liberated  by  a force  which  the  surrounding  mass  has  transmitted  uuthout  prejudice  to 
its  solidity.  Supposing,  for  example,  that  solidity  is  limited  by  molecular  ’vibrations  of 
a certain  amplitude,  those  at  the  surface  of  the  internal  cavity  may  exceed  this,  while 
those  between  the  cavity  and  the  external  surface  of  the  ice  may,  by  their  reciprocal 
actions,  be  preserved  within  it,  just  as  the  terminal  member  of  a series  of  elastic  balls  is 
detached  by  a force  which  has  been  transmitted  by  the  other  members  of  the  series 
without  visible  separation'*. 
35.  Where,  however,  experiment  is  within  reach,  we  ought  not  to  trust  to  specula- 
tion ; and  I was  particularly  anxious  to  obtain  an  unequivocal  reply  to  the  question 
whether  an  interior  portion  of  a mass  of  ice  could  be  melted  by  heat  which  had  passed 
through  the  substance  by  the  process  of  conduction.  A piece  of  Norway  ice,  containing 
a great  number  of  the  liquid  disks  already  described,  and  several  cells  of  aii’  and  water, 
was  enveloped  in  tinfoil  and  placed  in  a mixture  of  pounded  ice  and  salt.  A few  minutes 
sufficed  to  freeze  the  disks  to  thin  dusky  circles,  which  appeared,  in  some  cases,  to  be 
formed  of  concentric  rings,  and  reminded  me  of  the  sections  of  certain  agates.  Looked 
at  sideways,  these  disks  were  no  thicker  than  a fine  line.  The  water-cells  were  also 
frozen,  and  the  associated  air-bubbles  were  greatly  diminished  in  size.  I placed  the 
mass  of  ice  between  me  and  a gas-light,  and  observed  it  through  a lens:  after  some 
time  the  disks  and  water-cells  showed  signs  of  breaking  up  again.  The  rings  of  the 
disks  disappeared ; the  contents  seemed  to  aggregate  so  as  to  form  larger  liquid  spots, 
and  finally,  some  of  them  were  reduced  to  clear  transparent  disks  as  before. 
36.  But  an  objection  to  this  experiment  is,  that  the  ice  may  have  been  liquefied  by 
the  radiation  from  the  lamp,  and  I have  experiments  to  describe  which  will  show  the 
justice  of  this  objection.  A rectangular  slab,  1 inch  thick,  3 inches  long  and  2 nide, 
was  therefore  taken  from  a mass  of  Norway  ice,  in  which  the  associated  air-  and  water- 
cells  were  very  distinct.  I enveloped  it  in  tinfoil  and  placed  it  in  a freezing  mixture. 
In  about  ten  minutes  the  water-blebs  were  completely  frozen  within  the  mass.  It  was 
immediately  placed  in  a dark  room,  where  no  radiant  heat  could  possibly  affect  it,  and 
examined  every  quarter  of  an  hour.  The  dim  frozen  spots  gradually  broke  up  into  little 
water  parcels,  and  in  two  hours  the  water-blebs  were  perfectly  restored  in  the  centre  of 
the  slab  of  ice.  When  last  examined,  this  plate  was  half  an  inch  thick,  and  the  drops 
of  liquid  were  seen  right  at  its  centre. 
* Of  course  I intend  this  to  help  the  conception  merely. 
