Am.  Jour.  Pharm. 
December,  1895. 
Tubular  Prismatic  Crystals. 
603 
crystals  developed.  The  presence  of  sodium  hydroxide  in  a  solu- 
tion of  sodium  chloride  may  cause  the  latter  to  crystallize  in  octa- 
hedra  instead  of  the  usual  cubes,  and  the  presence  of  the  same 
agent  in  a  concentrated  solution  of  common  alum  conduces  to  the 
formation  of  cubes  instead  of  the  customary  octahedra. 
Fig.  i. 
Fig. 
It  frequently  happens  that  a  well-developed  crystal  can  be  seen 
within  a  larger  crystal,  both  being  of  the  same  form  and  composi- 
tion.   Occasionally,  we  meet  one  form  of  crystal  enveloping  another 
B 
Fig.  3. 
form.  Prof.  Williams,  in  his  admirable  work  on  crystallography, 
cites  a  crystal  of  calcite  of  rhombohedral  habit,  in  the  centre  of 
which  can  plainly  be  seen  a  darker  scalenohedron  {Fig.  i). 
The  cavernous  crystals  {Fig.  2),  or  "  hoppers,"  as  they  are  some- 
times called,  present  another  singular  form  of  crystallization.  These 
