216 
PEOFESSOE  TYKDALL  ON  SOME  PHYSICAL  PEOPEETIES  OP  ICE. 
during  his  earlier  investigations  on  the  glacier  of  the  Aar.  The  same  appearances  have 
been  described  by  the  Brothers  Schlagintweit,  and  finally  attention  has  been  forcibly 
draAvn  to  the  subject  in  a recent  paper  by  Mr.  Huxley,  published  in  the  Philosophical 
Magazine*. 
22.  The  only  explanation  of  this  phenomenon  hitherto  given,  and  adopted  apparently 
without  hesitation,  is  that  of  M.  Agassiz  and  the  Messrs.  Schlagixttveit.  These 
observers  attribute  the  phenomenon  to  the  diathermancy  of  the  ice  which  permits  the 
radiant  heat  to  pass  through  the  substance,  to  heat  the  bubbles  of  air,  and  cause  them 
to  melt  the  surrounding  icef. 
The  apparent  simplicity  of  this  explanation  contributed  to  ensure  its  general  accept- 
ance ; and  yet  I think  a little  refiection  will  show  that  the  hypothesis,  simple  as  it  may 
appear,  is  attended  with  grave  difficulties. 
23.  For  the  sake  of  distinctness  I will  here  refer  to  a most  interesting  fact,  obseiwed 
first  by  M.  Agassiz,  and  afterwards  by  the  Messrs.  Schlagixtweit.  In  the  ‘ Systeme 
Glaciaire  ’ it  is  described  in  these  words : “ I ought  also  to  mention  a singular  property 
of  those  air-bubbles,  which  at  first  struck  us  forcibly,  but  which  has  since  received  a 
very  satisfactory  explanation.  When  a fragment  containing  air-bubbles  is  exposed  to 
the  action  of  the  sun,  the  bubbles  augment  insensibly.  Soon,  in  proportion  as  they 
enlarge,  a transparent  drop  shows  itself  at  some  point  of  the  bubble.  This  drop,  in 
enlarging,  contributes,  on  its  part,  to  the  enlargement  of  the  cavity,  and  following  its 
progress  a little,  it  finishes  by  predominating  over  the  bubble  of  ah’.  The  latter  then 
swims  in  the  midst  of  a zone  of  water  and  tends  incessantly  to  reach  the  most  elevated 
point,  at  least  if  the  flatness  of  the  cavity  does  not  hinder  it.” 
24.  The  satisfactory  explanation  here  spoken  of  is  that  ah’eady  mentioned:  let  us 
now  endeavour  to  follow  the  hypothesis  to  its  consequences.  Comparing  equal  weights 
of  both  substances,  the  specific  heat  of  water  being  1,  that  of  air  is  0*25.  Hence  to 
raise  a pound  of  water  one  degree  in  temperature,  a pound  of  ah  would  have  to  lose 
four  degrees. 
25.  Let  us  next  compare  equal  volumes  of  the  substances.  The  specific  graHty  of 
water  being  1,  that  of  air  is  yyo  > hence  a pound  of  ah  is  770  times  the  volume  of  a 
pound  of  water ; and  hence  for  a quantity  of  ah  to  raise  its  own  volume  of  water  1 degree, 
it  must  part  with  770  X 4,  or  3080  degrees  of  temperature. 
* October,  1857. 
t II  est  evident  pour  qniconque  a suivi  le  progres  de  la  physique  moderne,  que  ce  plienomeue  est  du 
uniquement  a la  diathermaneite  de  la  glace  (Agassiz,  Systeme,  page  157). 
Das  Wasser  ist  dadurch  enstanden  dass  die  Liift  Warmestrahlen  absorbirte  welcbe  das  Eis  als  diatber- 
maner  Korper  durcbliess  (Schlagintweit,  IJntersucbungen,  S.  17). 
X Page  168.  The  figures  to  which  M.  Agassiz  refers  in  the  note  to  this  page  seem  to  be  eorrectly 
drawn,  but  his  descriptive  reference  to  them,  though  in  part  correct,  is  in  part  unintelligible  to  me.  He 
uses  the  term  tulles  for  the  bubbles  of  air,  and  gouttelettes  for  the  drops  of  water,  and  I beheve  the  latter 
term  is  always  restricted  to  a liquid.  But  if  we  restrict  the  term  thus  throughout  the  passage  in  question, 
there  is  no  escape  from  Mr.  Huxley’s  conclusion,  that  M.  Agassiz  has  taken  the  afi-bubbles  for  drops  of 
water,  and  the  drops  of  water  for  air-bubbles. 
