604 
Tubular  Prismatic  Crystals. 
{Am.  Jour.  Pharm. 
I>ecember,  1895. 
skeleton  forms  tend  to  develop  when  the  crystals  grow  rapidly. 
Halite,  galena  and  potassium  iodide  are  commonly  met  with  in  this 
form.  Mr.  Foote,  of  this  city,  also  showed  me  a  shallow,  cup-like 
crystal  of  vanadinite,  which  was,  indeed,  phenomenal.  Irregulari- 
ties of  surfaces  are  often  produced  by  corrosion  subsequent  to  the 
formation  of  the  crystals. 
Hollow  crystals  of  the  following  nature  are  occasionally  met 
with  :  A  crystal  develops  around  a  foreign  substance  of  a  less 
resisting  nature.  This  foreign  substance,  in  process  of  time,  is 
removed  by  chemical  or  physical  agents,  leaving  a  hollow  crystal. 
Quartz  crystals  of  this  nature  have  been  found. 
Thus,  we  have  examples  of  one  crystal  developing  around 
another,  crystals  whose  faces  are  cavernous,  irregularities  caused  by 
corrosion,  and  hollow  crystals,  formed  as  described  above  ;  but,  to 
my  knowledge,  no  geometrical,  tubular  crystals — crystals  that  have 
