Am-JJa°nuy-1^arm-}  Mayer's  Reagent  for-  Estimating  Alkaloids.  5 
In  general,  it  appears  that  the  formulas  must  be  less  simple  than  is 
commonly  represented,  and  they  show  a  smaller  proportion  of  both 
mercury  and  iodine  than  one  should  expect,  unless  we  abandon  the 
theory  that  the  compounds  are  double  iodides.  [It  will  be  remem- 
bered that  ammonia  is  precipitated  (in  alkaline  solutions  only)  as  a 
compound  of  quite  a  different  constitution,  the  formula  of  which  is 
Hg4JSr2I2,2H20.]  In  several  instances  the  proportion  of  iodine  is  but 
little  more  than  sufficient  to  satisfy  the  demands  of  the  dyad  mercury 
alone. 
The  question  of  vital  consequence  is  whether  precipitates  produced 
by  Mayer's  reagent  are  of  so  constant  a  composition  as  to  be  available 
for  purposes  of  gravimetric  estimations.  With  a  few  exceptions,  I  be- 
lieve that  where  approximate  results  only  are  contemplated,  they  are 
thus  available.  When  they  are  to  be  used  in  this  way,  they  must  be 
produced  by  adding  at  once  sufficient  reagent  to  precipitate  the  alka- 
loid completely,  with  a  small  margin  of  excess.  The  precipitate  must 
be  allowed  to  stand  several  hours  before  it  is  collected.  It  will  not 
bear  much  washing.  Sometimes  it  will  adhere  firmly  to  the  beaker. 
When  this  is  the  case  it  may  be  washed  once  or  twice  superficially 
with  water,  dried  in  the  beaker  and  weighed.  Otherwise  it  can  be 
best  collected  on  a  pair  of  mutually  counterpoised  filters,  washed  with 
a  little  water,  so  applied  as  to  wash  the  filters  rather  than  the  pre- 
cipitate, pressed  between  folds  of  filter  paper,  dried  at  100°  C,  and 
weighed.  It  is  evident  that  a  precipitate  thrown  down  in  a  solution 
heavily  loaded  with  foreign  substances,  such  as  that  obtained  from  a 
fluid  extract,  cannot  be  advantageously  used  in  this  mode  of  estima- 
tion, since  the  precipitate  would  carry  down  mechanically  too  much 
foreign  matter,  which  we  cannot  wash  out  without  material  loss  of 
the  substance  of  our  precipitate.  It  is  therefore  advisable  in  such  a 
case  to  separate  the  alkaloid,  in  a  crude  form  at  least,  in  the  first  in- 
stance, dissolve  it  in  a  little  acid,  make  up  to  a  suitable  volume — the 
solution  should  not  contain  more  than  1 : 200  of  the  alkaloid — and 
precipitate  from  this  prepared  solution. 
Further  experiments  are  necessary  to  ascertain  exactly  what,  under 
the  conditions  here  prescribed,  will  be  the  average  weight  of  the  pre- 
cipitates produced. 
The  principal  reason  why  the  results  of  titrations  made  under 
varying  conditions  show  such  large  differences,  is  that  there  is  always 
required  to  complete  the  precipitation  of  the  alkaloid  a  notable  excess 
