8       CHEMICAL  COMPOSITION  OF  GELSEMIUM  SEMPERVIRENS. 
cipitate  is  readily  soluble  in  free  acids,  and  its  nature  may  be 
confirmed  by  addition  of  excess  of  nitric  acid  and  then  of 
ammonia. 
Nitrate  of  Suboxide  of  Mercury  also  precipitates  the  acid  in 
the  form  of  a  dirty  yellow  deposit. 
3.  Nitrate  of  Silver  produces  in  solution  of  the  acid  a  yellow 
or  brownish-yellow  precipitate,  which  slowly  acquires  a  nearly  or 
altogether  black  color  and  is  then  insoluble  in  nitric  acid.  The 
1-lOOth  of  a  grain  of  the  acid  yields  a  very  copious  precipitate. 
1-lOOOth  grain  yields  at  first  only  a  faint  turbidity,  but  in  a 
little  time  there  is  a  quite  copious  black  or  bluish-black  precipi- 
tate. l-10,000th  grain  will  yield  after  some  minutes  a  good 
black  deposit ;  and  after  several  minutes,  one  drop  of  a  50-OOOth 
solution  of  the  acid  will  acquire  a  distinct  purplish  or  blackish 
color. 
4.  Sulphate  of  Copper  throws  down  from  tolerably  strong 
solutions  of  the  acid  a  brownish-red  precipitate,  which  quickly 
acquires  a  dull  red  color,  and  after  a  time  becomes  partly  gran- 
ular and  crystalline.  The  precipitate  is  readily  decomposed  by 
free  acids  with  the  elimination  of  the  organic  acid. 
5.  Sulphate  of  Iron  produces  in  solutions  of  the  acid,  when 
not  too  dilute,  a  black  precipitate  which  quickly  becomes  brown, 
and  after  a  time  masses  of  colorless  crystalline  needles  appear. 
6.  Chloride  of  Gold  occasions  a  deep  green  precipitate,  quickly 
becoming  bluish  and  appearing  black  by  reflected  light.  The 
precipitate  is  insoluble  in  acetic  acid.  1-lOOOth  of  a  grain  of 
the  acid  yields  a  good  bluish  deposit. 
7.  Bichloride  of  Platinum  produces,  in  strong  solutions  of  the 
acid,  a  dirty  yellow  amorphous  precipitate,  which  is  insoluble  in 
acetic  acid,  and  after  a  time  becomes  granular. 
8.  A  solution  of  bromine  in  bromohydric  acid  throws  down 
from  a  drop  of  a  100th  solution  of  the  acid  a  copious  greenish 
precipitate,  which  quickly  acquires  a  bluish,  then  a  dark  grey 
color.  One  drop  of  a  1,000th  solution  yields  a  decided  green 
precipitate,  which  finally  acquires  a  deep  blue  color. 
9.  Iodine  in  solution  of  Iodide  of  Potassium  produces  in  so- 
lutions of  the  acid,  when  not  very  dilute,  a  copious  reddish- 
