48 
CRYSTALS  IN  THE  BARK  OF  GUAIACUM. 
portion,  and  may  therefore  be  readily  recognized  upon  the  thin 
lamellae  into  which  the  bark  may  be  split  after  a  short  macera- 
tion in  water  ;  ea^h  of  the  cells  of  the  parenchyma,  which  are 
placed  in  very  regular  rows,  contains  a  single  crystal  which  occu- 
pies nearly  their  whole  interior. 
From  thin  longitudinal  cuts,  some  of  the  crystals  may  be 
easily  separated  by  ablution,  and  on  rolling  them  about  under 
the  microscope,  they  are  easily  recognized  as  four-sided  prisms, 
and  as  twin-crystals  of  the  same  size.  The  presence  of  these 
so-called  swallow-tailed  crystals  led  to  the  supposition  of  their 
being  gypsum,  and  this  was  verified  by  the  micro-chemical 
analysis. 
If  the  crystals  are  heated,  they  assume  a  milk-white  color, 
which  is  characteristic  of  the  crystals  of  sulphate  of  lime.  A 
similar  change,  under  somewhat  different  conditions,  may  be 
observed,  if  a  very  thin  longitudinal  section  of  this  interior 
bark  in  a  tangential  direction  is  incinerated.  If  it  is  carefully 
moistened  with  water,  it  will  be  observed  that  the  crystals  have 
changed  their  color  to  a  milk-white,  but  have  remained  in  their 
respective  positions.  Reduced  to  sulphide  of  calcium  by  the  car- 
bon of  the  cellulose,  they  are  dissolved  with  effervescence  on  the 
addition  of  nitric  acid,  and  the  evaporated  solution  again  yields 
prisms  of  sulphate  of  lime  in  star-like  groups,  but  consider- 
ably smaller  in  size. 
The  same  twin-crystals  of  gypsum  are  likewise  found  in  the 
inner  bark  of  other  plants ;  for  instance,  in  the  bark  of  Swietenia 
Senegalensis,  and  in  a  somewhat  different  shape  in  the  bark  of 
Pterocarpus  Massupium.  The  crystals  of  gypsum  in  the  parenchy- 
mous  cells  of  the  various  species  of  Musa  are  of  various  shapes, 
and  are  always  found  in  a  larger  number  in  the  cells,  which  are 
never  entirely  filled  by  them  Archiv  d.  Pharm.  1859,  August 
155,  156. 
