564  PEOF.  J.  W.  JUDD  OX  COMPOSITE  DYKES  IX  AEEAX.  [Nov.  1 893. 
therm odynamical  principles,  there  should  he — in  a  very  great  volume 
of  a  solution,  especially  if  near  saturation — a  tendency  for  conceutra- 
tion  to  take  place  in  the  lower  parts  of  the  mass.  Such  vast  masses 
of  molten  silicates  as  must  exist  in  the  deeper  parts  of  the  earth’s 
crust  are  precisely  the  kind  of  solutions  in  which  we  may  expect  to 
find  the  action  of  this  law  illustrated. 
There  is  still  another  physical  principle  which  has  been  appealed 
to  as  affording  an  explanation  of  the  differentiation  which  takes 
place  in  a  liquefied  mass  of  silicates.  In  the  same  year  that  Gouy 
and  Chaperon  demonstrated  the  causes  that  would  lead  to  “  the 
concentration  of  solutions  by  gravity,”  van’t  Hoff  published  his 
paper  on  ‘  The  Hole  of  Osmotic  Pressure  in  the  Analogy  between 
Solutions  and  Gases.’ 1  He  argued  that  solutions  must  follow  the 
law  of  gaseous  tension,  and  that  if  two  parts  of  a  solution  be  main¬ 
tained  at  different  temperatures,  concentration  must  take  place  in 
the  cooler  part.  It  is  interesting  to  note  that,  six  years  before 
van’t  Hoff  published  his  general  conclusions,  Soret  had  conducted 
a  series  of  experimental  researches  which  showed  that,  when  two 
portions  of  a  solution  are  maintained  at  different  temperatures, 
there  is  always  a  tendency  to  concentration  in  the  colder  part.2 
If,  as  we  may  assume  is  generally  the  case,  the  lowest  portions 
of  the  great  liquid  reservoirs  within  the  earth’s  crust  are  at  the 
highest  temperature,  then  the  principles  established  by  Gouy  and 
Chaperon  and  by  van’t  Hoff  respectively  would  operate  in  contrary 
directions  and  tend  to  neutralize  each  other.  It  is  sufficient  for 
our  present  purpose,  however,  to  call  attention  to  the  fact  that 
there  are  now  well-recognized  physical  principles,  in  accordance 
with  which  differentiation  must  necessarily  he  set  up  in  the  heated 
solutions  constituted  by  molten  masses  of  mixed  silicates,  anterior 
to,  and  independently  of,  the  liquation  that  may  follow  selective 
crystallization.  That  differentiation  does  take  place  before,  as  well 
as  during,  crystallization,  the  phenomena  exhibited  by  the  two 
classes  of  Composite  Dykes  afford  interesting  and  striking  proofs. 
[At  the  recent  Meeting  of  the  British  Association  at  Nottingham, 
Mr,  Alfred  Harker  suggested  that  there  is  another  plrysical  principle 
which  may  he  appealed  to  as  explaining  the  differentiation  which 
takes  place  in  homogeneous  molten  magmas.  This  is  Berth elot’s 
‘  principle  of  maximum  work,’  or  the  cognate  one  of  ‘  most  rapid  de¬ 
gradation.’  Migration  of  the  least  soluble  ingredient  to  the  part  of 
the  liquid  most  easily  saturated  would  determine  crystallization, 
the  process  which  would  give  rise  to  the  most  rapid  evolution  of 
heat. — September  80th,  1893.] 
1  Ztitsekr.  f.  Pkys.  Cheraie,  vol.  i.  (1887)  p.  481. 
2  Ann.  dfc  Ckimie  et  de  Physique,  5th  ser.  vol.  xxii.  (1881)  p.  293. 
