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XIII.  On  some  Physical  Properties  of  Ice.  By  John  Tyndall,  F.B.S.,  Professor  of 
Natural  Philosophy  in  the  Royal  Institution  of  Great  Britain. 
Eeceived  December  17, — Bead  December  17,  1857. 
The  observations  upon  Glaciers,  to  be  recorded  in  a subsequent  paper,  led  me  to  make 
some  experiments  upon  the  physical  properties  of  ice,  the  results  of  which  are,  I hope, 
of  sufficient  interest  to  justify  me  in  presenting  them  to  the  Eoyal  Society. 
§1- 
1.  I availed  myself  of  the  fine  sunny  weather  with  which  we  were  favoured  last  Sep- 
tember and  October,  to  examine  the  efiects  of  solar  heat  upon  ice.  The  experiments 
were  made  with  the  Wenham  Lake  and  Norway  ice.  Slabs  were  formed  of  the  sub- 
stance, varying  from  one  to  several  inches  in  thickness,  and  these  were  placed  in  the 
path  of  a beam  rendered  convergent  by  a double  convex  lens  4 inches  in  diameter,  and 
possessing  a focal  distance  of  10-|^  inches.  The  slabs  were  usually  so  placed,  that  the 
focus  of  parallel  rays  fell  within  the  ice.  Having  first  found  the  position  of  the  focus  in 
air,  the  lens  was  screened ; the  ice  was  then  placed  in  position,  the  screen  was  removed, 
and  the  efiect  was  watched  through  an  ordinary  pocket  lens. 
2.  A plate  of  ice  an  inch  thick,  with  parallel  sides,  was  first  examined:  on  removing 
the  screen  the  transparent  mass  was  crossed  by  the  sunbeams,  and  the  path  of  the  rays 
through  it  was  instantly  studded  by  a great  number  of  little  luminous  spots,  produced 
at  the  moment  and  resembhng  shining  air-bubbles.  When  the  beam  was  sent  through 
the  edge  of  the  plate,  so  that  it  traversed  a considerable  thickness  of  the  ice,  the  path 
of  the  beam  could  be  traced  by  those  brilliant  spots,  as  it  is  by  the  fioating  motes  in  a 
dark  room. 
3.  In  lake  ice  the  planes  of  freezing  are  easily  recognized  by  the  stratified  appear- 
ance which  the  distribution  of  the  air-bubbles  gives  to  the  mass.  A cube  was  cut  from 
a perfectly  transparent  portion  of  the  ice,  and  the  solar  beam  was  sent  through  the  cube 
in  three  rectangular  directions  successively.  One  was  perpendicular  to  the  plane  of 
fr’eezing,  and  the  other  two  parallel  to  it.  The  bright  bubbles  were  formed  in  the  ice 
in  all  three  cases. 
4.  When  the  surfaces  perpendicular  to  the  planes  of  freezing  were  examined  by  a lens, 
after  exposure  to  the  light,  they  were  found  to  be  cut  up  by  innumerable  small  parallel 
fissures,  with  here  and  there  minute  spurs  shooting  from  them,  which  gave  the  fissures, 
in  some  cases,  a feathery  appearance.  When  the  portions  of  the  ice  traversed  by  the 
beam  were  examined  parallel  to  the  surface  of  freezing,  a very  beautiful  appearance 
2 F 2 
