CRYSTALLIZATION  OF  SUPERSATURATED  SOLUTIONS,  ETC.  379 
Specific  gravity.  Seconds. 
Paraffineoil,  ...  797  286 
Petroleum,  ...       797  375 
These  experiments  I  did  not  mean  for  publication,  but  only  as 
preliminary  to  a  larger  and  more  accurate  series  I  intended 
making,  with  tubes  of  as  exactly  uniform  bore  as  could  be  ob- 
tained, and  on  a  considerable  variety  of  liquids  ;  but  I  must  give 
up  the  subject,  at  least  for  the  present,  on  account  of  my  distance 
from  a  situation  where  these  can  be  conveniently  obtained. 
Mandal,  Norway. 
— London  C7iem.  Neivs,  March  31,  1865. 
ON  THE  CRYSTALLIZATION  OF  SUPERSATURATED  SA- 
LINE SOLUTIONS,  AND  THE  NORMAL  PRESENCE  OF 
SULPHATE  OF  SODA  IN  THE  AIR. 
By  M.  D.  Gernez. 
The  crystallization  of  supersaturated  saline  solutions  is  one  of 
the  phenomena  which  has  most  taxed  the  ingenuity  of  chemists 
to  explain ;  the  uncertain  circumstances  under  which  it  is  pro- 
duced, the  variety  of  causes  which  appear  to  determine  or  pre- 
vent it,  and  the  hypotheses  put  forward,  have  all  contributed  to 
increase  the  importance  of  this  subject. 
Gay-Lussac  has  shown  that  a  solution  of  sulphate  of  soda  will 
remain  at  the  ordinary  temperature  without  crystallizing,  even 
when  it  contains  several  times  the  weight  of  the  salt  which  it 
dissolves  at  this  same  temperature,  but  that  it  forms  into  a  mass 
when  a  crystal  of  sulphate  of  soda  or  certain  substances  pre- 
viously exposed  to  the  air  are  thrown  into  it,  or  when  the  liquid 
is  abruptly  placed  in  contact  with  the  air.  Many  chemists 
(Loewel,  Selmi,  Goskinski,  &c.)  have  varied  and  extended  Gay- 
Lussac's  experiments  ;  they  have  brought  to  bear  on  the  expla- 
nation of  this  phenomenon  sometimes  water  vapor,  sometimes 
atmospheric  air,  or  an  unknown  substance,  or  glass  vessels  in  a 
particular  state,  or  finally  a  catalytic  force.  I  believe  I  have 
shown  that  the  crystallization  of  a  supersaturated  solution  of 
sulphate  of  soda  is  effected  by  contact  with  a  fragment,  efflores- 
cent or  not,  of  sulphate  of  soda,  of  ten  equivalents  of  water. 
This  appears  to  me  to  be  the  result  of  a  number  of  experiments 
of  which  I  can  here  indicate  only  the  most  important. 
