Gas  or  Vapour  from  its  Supersaturated  Solution.        207 
over  with  minute  points  of  carbon  &c,  which  act  as  excellent 
nuclei  and  save  many  a  vessel  from  destruction.  M.  Solaro 
makes  me  say  that  these  are  points  which  the  eye  cannot  detect ; 
whereas  I  say  no  such  thing.  Any  one  who  boils  liquids  in 
glass  vessels  must  have  noticed  ascending  vortices  of  vapour 
from  certain  points  in  the  glass,  and  that  these  points  consist  of 
small  black  specks  of  carbon  which  act  as  excellent  nuclei. 
Hence  it  cannot  be  said  of  these,  u  lorsque  tout  Fair  ou  le  gaz  a 
disparu,  le  phenomene  a  cesse,  mais  seulement  alors  et  par  cette 
raison."  Any  one  of  these  points  will  remain  active  during 
many  hours,  and  surely  it  cannot  be  seriously  maintained  that 
during  all  tbis  time  it  is  giving  off  air. 
if.  Solaro  asks,  "  Pouvons-nous  supposer  qu'un  corps  quel- 
conque  se  purifie  par  le  simple  contact  d'un  liquide  chaud  quel 
qu'il  soit?"  The  action  of  liquids  at  the  boiling-point  in  ren- 
dering bodies  chemically  clean  ought  to  be  well  known  to  every 
one  who  has  distilled  sulphuric  acid  or  alcoholic  or  ethereal 
liquids,  were  it  not  that  the  idea  is  so  strongly  fixed  in  the  mind 
that  all  "promoters  of  vaporization  "  act  by  carrying  down  air. 
Sand,  cleaned  by  boiling  in  sulphuric  acid,  washing,  and  heating, 
or  clean  mercury  (as  described  in  my  experiments)  introduced  into 
boiling  water,  must  surely,  according  to  the  theory  advocated  by 
M.  Solaro,  carry  down  air;  but  they  produce  soubresauts  at 
once ;  whereas  a  little  unclean  sand  or  unclean  mercury  stops 
the  soubresauts  and  produces  tranquil  boiling. 
The  permanent  nuclei,  such  as  charcoal  and  other  porous 
bodies,  act,  as  I  believe,  on  Saussure's  principle  of  the  absorption 
of  gases  and  vapours.  A  piece  of  charcoal  will  act  for  many 
hours,  and  even  days,  in  liberating  vapour  from  a  boiling  liquid ; 
and  can  it  be  supposed  for  a  moment  that  during  all  this  time 
it  is  giving  off  air  as  well  as  vapour  ? 
If  there  are  any  other  points  in  M.  Solaro's  objections  that  I 
have  not  noticed,  it  is  because  I  am  unwilling  to  repeat  what 
has  been  already  said  in  my  two  papers.  In  the  second  paper 
I  have  pointed  out  the  different  results  obtained  as  to  tempera- 
ture by  heating  a  liquid  by  means  of  a  flame  applied  below,  and 
by  placing  a  vessel  in  a  bath  of  hot  oil  or  other  source  of  heat. 
I  have  als'j  shown  how  very  improbable  it  is  that  gases  such  as 
nitrogen,  which  are  so  little  soluble  in  water,  especially  in  boil- 
ing water,  should  continue  to  exist  in  it  after  long  boiling.  I 
have  often  repeated  Mr.  Grove's  experiment,  in  which  water 
covered  with  oil  was  repeatedly  boiled.  I  covered  the  water 
with  paraffine-oil  and  boiled  it  several  times  a  day  during  a 
week.  The  result  was,  that,  if  the  boiling  be  somewhat  brisk, 
the  oil  becomes  broken  up  into  globules,  which  are  carried 
down  to  the  bottom  of  the  tube.  If  less  brisk,  the  surface 
of  the  oil  opens,  lets  in  air  and  closes  upon  it ;  the  air  then 
