PROFESSOE  TY^^DALL  OF  SOME  PHYSICAL  PROPERTIES  OF  ICE. 
217 
26.  Now  the  latent  heat  of  water  is  142°' 6 Fahr.,  hence  the  quantity  of  heat  required 
to  melt  a certain  weight  of  ice  is  142 ‘6  times  the  quantity  required  to  raise  the  same 
weight  of  water  1 degree  in  temperatm-e ; hence  a measure  of  air,  in  order  to  reduce  its 
o^vn  volume  of  ice  to  the  liquid  condition,  must  lose  3080  X 142'6,  or  439,208  degrees  of 
temperature. 
27.  This  then  gives  us  an  idea  of  the  amount  of  heat  which,  according  to  the  above 
hypothesis,  is  absorbed  by  the  bubble  and  communicated  to  the  ice  during  the  time 
occupied  in  melting  a quantity  of  the  latter  equal  in  volume  to  the  bubble,  which  time 
is  stated  to  be  brief ; that  is  to  say,  the  quantity  of  heat  supposed  to  be  absorbed  by  the 
air  would,  if  it  had  not  been  communicated  to  the  ice,  have  been  sufficient  to  raise  the 
bubble  itself  to  a temperature  160  times  that  of  fused  cast  iron.  Had  air  this  power  of 
absorption,  it  might  be  attended  with  inconvenient  consequences  to  the  denizens  of  the 
earth ; for  we  should  dwell  at  the  bottom  of  an  atmospheric  ocean,  the  upper  strata  of 
which  would  effectually  arrest  all  calorific  radiation. 
28.  It  is  established  by  the  experiments  of  Delaeoche  and  Melloni*,  that  a calorific 
beam,  emerging  from  any  medium  which  it  has  traversed  for  any  distance,  possesses,  in 
an  exalted  degree,  the  power  of  passing  through  an  additional  length  of  the  same  sub- 
stance. Absorption  takes  place,  for  the  most  part,  in  the  portion  of  the  medium  first 
traversed  by  the  rays.  In  the  case  of  a plate  of  glass,  for  example,  VI \ per  cent,  of  the 
heat  proceeding  from  a lamp  is  absorbed  in  the  first  fifth  of  a millimetre  ; whereas,  after 
the  rays  have  passed  through  6 milhmetres  of  the  substance,  an  additional  distance  of  2 
millimetres  absorbs  less  than  2 per  cent,  of  the  rays  thus  transmitted.  Supposing  the 
rays  to  have  passed  through  a plate  25  millimetres,  or  an  inch  in  thickness,  there  is  no 
doubt  that  the  heat  emerging  from  such  a plate  would  pass  through  a second  layer  of 
glass,  1 millimetre  thick,  without  suffering  any  measureable  absorption.  For  an  incom- 
parably stronger  reason,  the  quantity  of  solar  heat  absorbed  by  a bubble  of  air  at  the 
earth’s  surface,  after  the  rays  have  traversed  the  whole  thickness  of  our  atmosphere,  and 
been  sifted  in  their  passage  through  it,  must  be  wholly  inappreciable. 
29.  To  the  sifting  power  of  the  atmosphere  we  must  add,  in  the  case  of  the  glacier, 
the  absorptive  powder  of  the  ice.  Some  notion  of  this  power,  as  compared  with  that  of  air, 
may  be  gathered  from  the  folio-wing  facts : — As  regards  the  variation  of  the  intensity  of 
radiant  heat  wfith  the  distance,  the  law  of  inverse  squares  is  capable  of  the  strictest  expe- 
rimental verification  in  air,  even  when  the  source  of  heat  is  far  below  212*^  Fahr. 
This  implies  that  the  absoi’ption  in  the  space  of  air  through  which  the  heat  passes  is 
too  small  to  disturb  the  harmony  of  the  law.  Now  a plate  of  ice,  one-tenth  of  an  inch 
thick,  is  absolutely  imper-vious  to  heat  emanating  from  a source,  not  only  of  212°,  but 
of  752°  Fahr.  ; and  is  capable,  moreover,  of  absorbing  99^  per  cent,  of  the  calorific  rays 
emitted  by  an  incandescent  platinum  wiref.  (La  Thermochrose,  p.  164.) 
* La  Thermochrose,  p.  202. 
t I can  hardly  forbear  drawing  attention  here  to  the  remarkable  influence  which  the  element  hy(h'ogen 
appears  to  exercise  upon  radiant  heat,  and  the  longer  waves  of  light.  Wherever  hydrogen  enters  into  a 
MDCCCLVIII.  2 G 
