OBSERVATIONS  ON  ROCK  SALT. 
351 
The  process  was  then  shown.  It  consisted  in  placing  a  few 
drops  of  the  solution  in  a  suitable  box,  shaking  the  pills  in  it. 
and  then  turning  them  upon  a  slab,  and  so  placing  them  that 
one  pill  did  not  touch  another.  In  a  few  minutes  they  were 
quite  dry. 
OBSERVATIONS  ON  ROCK  SALT. 
By  M.  Margueritte. 
It  is  well  known  that  when  salt  is  fused,  it  crystallizes  on  cool- 
ing in  various  forms,  especially  in  cubes  ;  these  crystals  are  more 
or  less  confused,  opake,  and  always  colored  when  common  salt  or 
crude  rock-salt  is  employed.  The  results  are  different  when 
pure  salt  is  calcined,  kept  in  a  state  of  quiet  fusion,  and  cooled 
slowly ;  in  this  crystals  are  formed,  sometimes  of  considerable 
volume,  and  perfectly  transparent. 
When  protected  from  the  air,  rock-salt  in  its  native  state,  that 
is  to  say,  presenting  various  gray,  red  or  brown  tints,  may  be 
fused  without  decolorization ;  but  if  the  calcination  takes  place 
in  contact  with  the  air,  and,  as  in  the  preceding  case,  the  fusion 
is  tranquil  and  the  refrigeration  slow,  the  earthy  matters  are 
deposited  at  the  bottom  of  the  crucible,  the  chloride  of  magne- 
sium is  spontaneously  decomposed  in  contact  with  the  moist 
atmosphere,  the  coloring  matters  are  destroyed  under  the  oxidiz- 
ing action  of  the  air,  and  all  the  impurities  are  eliminated  by 
the  crystallization  which  takes  place  in  the  mass ;  in  this  way 
two  very  distinct  strata,  which  are  easily  separated,  are  formed 
in  the  mass.  This  operation  might  perhaps  be  applied  with  ad- 
vantage to  the  purification  of  crude  rock-salt,  and  also  to  that  of 
ordinary  sea-salt. 
The  fusion  of  salt,  effected  with  or  without  the  contact  of  the 
air,  would  explain,  up  to  a  certain  point,  how  the  salt  which  is 
found  in  the  bosom  of  the  earth  is  contaminated  with  coloring 
matters,  and  how,  on  the  contrary,  that  which  has  been  exposed 
to  an  oxidizing  atmosphere  is  white  and  transparent. 
Prom  these  facts  we  can  come  to  no  conclusion  as  to  the  origin 
and  formation  of  rock-salt ;  for  although  by  fusion  we  can  obtain 
salt  having  the  aspect,  the  transparency,  and  the  physical  pro- 
perties of  the  latter,  it  is  nevertheless  certain  that  the  presence 
