216  Preparation  of  Compressed  Tablets.  {Am'^;^rm' 
unalterable  fact.  You  may  change  the  form,  the  peculiar  con- 
struction of  the  punches  or  die  ;  but  so  long  as  the  face  of  them  pre- 
sents a  smooth  surface  to  the  material  to  be  compressed,  it  is 
always  the  same.  Remembering  this,  you  will  not  ascribe  the 
fault  to  the  die  or  punch,  if  your  material  adheres  to  them.  The 
punch  should  be  perfectly  smooth  and  have  sharp  edges,  and  move 
freely  in  the  dies.  They  should  be  made  of  tool  steel  and  tempered 
just  hard  enough  to  prevent  bending  under  pressure — beyond  this 
you  should  expect  nothing,  and  if  the  material  adheres  to  them,  you 
must  look  to  the  material  as  the  thing  at  fault.  As  a  rule,  you 
should  cause  the  cohesive  property  of  the  material  to  be'  greater 
than  the  adhesive,  and  when,  by  experiment,  you  find  where  the 
fault  is,  all  that  you  have  to  do  is  to  apply  your  knowledge  of  the 
nature  of  the  different  excipients  to  correct  it.  There  are  some 
materials  that  are  neither  cohesive  nor  adhesive;  for  instance,  if  an 
ounce  of  pulverized  charcoal  were  ordered  to  be  made  into  40  loz- 
enges, you  would  have  no  trouble  in  their  sticking  to  the  dies  and 
punches,  but  you  would  have  a  great  deal  in  getting  any  cohesion 
between  the  different  particles  of  the  material.  The  question  here 
would  be  to  add  something  that  would  cause  a  cohesion  greater 
than  adhesion,  and,  at  the  same  time,  not  destroy  the  effect  of  the 
charcoal  as  a  remedy.  Here  dextrin,  wax,  gelatin,  gum  arabic  and 
tragacanth,  mastich,  etc.,  present  themselves,  as  the  different  parti- 
cles of  the  charcoal  must  actually  be  glued  together. 
If  you  were  ordered  to  make  480  grains  of  salicylate  of  soda  into 
96  tablets,  you  might  add  some  pulv.  acacia,  dampen  with  alcohol 
and  water,  run  through  a  No.  XXX  sieve  and  dry.  Just  before  using, 
stir  in  some  talc  to  prevent  sticking.  There  are  other  ways,  with- 
out the  use  of  talc,  but  it  is  better  to  learn  this  way  first. 
The  coal  oil  products  will  claim  your  attention  very  often.  Most 
of  them  are  not  soluble  in  water,  and  when  pressed  alone  may  prove 
useless  on  account  of  their  insolubility.  A  small  quantity  of  starch 
added  to  the  mixture  may  often  become  of  great  service.  Say  you 
take  salol,  phenacetin,  starch  ;  dampen  with  alcohol,  run  through  a 
No.  XX  sieve,  shake  over  a  gas  jet  to  slightly  warm,  to  granulate 
and  dry  ;  a  moderate  heat  assists  in  granulating.  There  is  no  need 
of  anything  to  prevent  sticking. 
There  is  a  point  that  it  is  well  to  remember  :  Any  liquid  that  is 
not  a  solvent  to  any  of  the  ingredients  in  a  compound,  will  act  as  a 
