522 
CHEMICAL  NOTES. 
with  shorter  prismatic  crystals,  attached  to  one  another  in  the 
same  manner.  Among  these#  shorter  crystals  I  could  in  no  case 
discover  a  really  cubic  crystal. 
Occasionally,  I  have  met  among  the  last  crop  of  crystals  some 
which  had  all  the  appearance  of  two  or  three  planes  of  a  hollow, 
regular  octohedron ;  they  are  very  thin,  striated  lengthways 
under  the  magnifier,  as  if  composed  of  prisms,  and  look  like  a 
portion  of  the  hollow^  four-sided  pyramids  in  which  chloride  of 
sodium  frequently  crystallizes. 
It  appears  to  me,  in  accordance  with  the  foregoing  facts,  we 
shall  have  to  consider  chloride  of  potassium  as  dimorphous  in 
two  different  systems.  Besides  crystallizing  in  regular  octohe- 
drons,  the  solution  separates,  when  kept  near  its  boiling  point, 
regular  cubes,  and  on  and  after  cooling,  rectangular  prisms. 
Par  amor phism  of  the  Chlorides  of  Ammonium  and  Potassium. 
— The  salts  of  ammonia  are  isomorphous  with  the  corresponding 
salts  of  potassa.  If  a  solution  of  chloride  of  ammonium  is  evapo- 
rated and  allowed  to  crystallize,  feathery  crystals  are  obtained, 
which  on  closer  examination  prove  to  be  composed  of  small 
cubes.  If  chloride  of  ammonium  is  subjected  to  sublimation,  a 
fibrous  mass  is  gained,  which  evidently  has  a  crystalline  struc- 
ture, though  perfect  crystals  cannot  be  separated  from  it.  There 
seems  to  me  no  reason  why  these  fibres  ought  not  to  be  consid- 
ered as  prisms.  They  readily  split  in  the  direction  of  tjieir  long 
axis,  but  break  with  difficulty  transversely  ;  two  parallel  pris- 
matic planes  may  be  obtained  with  little  difficulty,  but  the  other 
two,  if  four-sided,  or  the  other  four  planes,  if  they  should  be  six 
sided  prisms,  are  usually  uneven  from  adhering  fibrous  crystals, 
and  cannot  be  readily  recognized.  From  analogy,  however,  we 
have  to  conclude  that  they  must  be  four-sided  prisms. 
The  prismatic  chloride  of  potassium  has  in  some  respects  a 
similar  behaviour.  It  is  not  so  tou^jh  as  the  ammonium-  salt, 
and  may  be  readily  reduced  to  powder ;  but  when  it  is  heated 
until  it  decrepitates,  the  prisms  chiefly  split  lengthwise,  just  like 
the  fibrous  crystals  of  chloride  of  ammonium  behave  under  the 
pestle  or  the  hammer. 
These  fibrous  crystals  cannot  belong  to  the  regular  system, 
for  the  feathery  crystals  which  are  made  up  of  regular  cubes, 
