218  Chemistry  of  a  Cup  of  Coffee.  {A%JaXi9Pilarm' 
easily  soluble  in  cold  water.  Subsequent  experiments  showed  that 
the  caffeine  in  coffee  is  combined  with  a  peculiar  acid  allied  possibly 
to  tannic  acid,  but  exhibiting  different  properties  from  the  tannin 
present  in  tea.  Thus  this  acid  (it  has  been  called  caffetannic  acid  by 
some  observers)  is  not  particularly  astringent,  has  a  sour  coffee-like 
taste,  does  not  coagulate  gelatin,  gives  a  light  green  colouration  with 
perchloride  of  iron  (whereas  tannic  acid  of  tea  turns  it  black),  does 
not  make  caffeine  solutions  thick  as  does  tannic  acid,  does  not  pre- 
cipitate alkaloids — e.g.,  quinine — and,  in  fact,  shows  altogether  dif- 
ferent properties  from  the  tannic  acid  of  tea.  It  gives  a  precipitate, 
however,  with  lead  acetate  from  which  the  acid  can  be  separated  by 
sulphuretted  hydrogen.  When  coffee  infusion  is  saturated  with 
ammonium  sulphate  a  precipitate  is  obtained  which  contains  a  small 
proportion  of  the  total  caffeine  in  the  free  state,  whereas  a  tea  infu- 
sion similarly  treated  gives  an  abundant  precipitate  containing  nearly 
all  of  the  caffeine,  this  precipitate  according  to  our  observations  con- 
sisting chiefly  of  caffeine  tannate. 
The  Absorption  of  Caffeine  Modified  by  its  Associates. 
The  caffeine  tannate  of  tea  is  precipitated  by  weak  acids,  and  the  * 
presumption  is  that  it  is  precipitated  by  the  gastric  juice,  and  there- 
fore the  caffeine  is  probably  not  absorbed  until  it  reaches  the  alkaline 
alimentary  tract.  In  the  case  of  coffee,  however,  in  whatever  form 
the  caffeine  may  be  present,  it  is  soluble  in  both  alkaline  and  acid 
fluids,  and  therefore  the  absorption  of  the  alkaloid  probably  takes 
place  in  the  stomach.  This  fact  may  have  an  important  physiological 
significance  when  we  consider  the  comparative  stimulating  effects  of 
the  two  beverages.  If  our  view  is  correct  coffee  should  act  more 
promptly  than  tea  as  a  stimulant  and  restorative,  looking  to  its 
physiological  action  as  due  for  the  most  part  to  caffeine.  In  prac- 
tice it  is  generally  accepted  that  coffee  is  a  more  powerful  restorative 
than  tea.  The  use  of  strong  coffee  as  an  antidote  in  poisoning  by 
narcotics,  notably  morphia,  is  of  interest  in  this  connexion.  Tea  is 
mentioned  for  the  same  purpose,  but  only  rarely.  Apart  from 
the  consideration  that  caffeine  has  probably  a  more  rapid  action 
when  taken  in  the  form  of  coffee  than  in  the  form  of  tea,  because 
the  caffeine  in  coffee  is  more  readily  absorbed,  it  must  also  be  re- 
membered that  coffee  is  often  made  with  a  generous  proportion  of 
the  powdered  bean,  as  in  the  case  of  the  after-dinner  "  black  "  coffee, 
