■604  Influence  of  Gum  Arabic  in  Reactions.     { ^'"lieT'issi'!'"'"' 
It  will  be  seen,  therefore,  that  under  the  conditions  stated  gum  pre- 
sented the  precipitation  of  metallic  sulphides.  In  concentrated  solu- 
tions, or  when  the  proportion  of  gum  was  very  small,  more  or  less 
incomplete  precipitations  were  observed. 
It  next  appeared  interesting  to  ascertain  whether  gum  has  also  the 
power  to  stop  the  precipitation  of  metallic  oxides  and  of  alkaloids  by 
the  ordinary  reagents.    The  following  were  the  results  obtained. 
As  to  the  metallic  oxides  it  is  unnecessary  to  enter  into  details  of 
the  experiments,  it  will  be  sufficient  to  say  that  they  behaved  like  the 
sulphides  in  the  presence  of  solution  of  gum.  But  since  the  oxides 
are,  as  a  rule,  much  more  soluble  in  water  than  the  corresponding  sul- 
phides, metallic  solutions  of  double  the  strength  were  used,  and  a  cor- 
responding solution  of  caustic  soda. 
The  action  of  gum  upon  the  formation  of  certain  other  precipitates 
was  also  studied,  the  following  being  the  results  observed : 
With  Gum.  Without  Gum. 
Neutral    phosphate  of    White  gelatinous  pre-    Clear  colorless  solution, 
ammonia  and  calcium  eipitate. 
chloride. 
Uranium   nitrate   and    Chocolate-brown  i)reci-    Clear  brown  solution, 
potassium     ferrocya-  pitate. 
nide. 
^ery  dilute  solution  of    Brown  gelatinous  pre-    Clear  light  brown  solu- 
ferric    chloride   and       eipitate.  tion. 
ammonia. 
In  a  like  manner  no  precipitates  were  produced  in  the  presence  of 
^um  in  solutions  of  1  in  1,000  of  the  following  alkaloids  :  quinine, 
cinchonine,  morphia,  strychnia,  brucia  and  veratria,  by  the  usual  alka- 
loidal  reagents — phosphomolybdate  of  ammonia,  double  iodide  of  mer- 
cury and  potassium  and  tannin.  If  it  be  remembered  that  the  sepa- 
ration of  alkaloids  by  their  reagents  has  to  be  effected  in  the  presence 
of  various  organic  matters  that  accompany  them  either  naturally  or 
accidentally,  these  researches  acquire  for  toxicology  an  unmistakable 
importance. 
It  is  necessary  also  to  note  that  certain  vegetable  infusions — among 
others  that  of  tea — are  opposed  to  the  precipitation  of  sulphide  of 
mercury.  The  liquid  only  acquires  a  darker  color,  consequent  upon 
the  presence  of  sulphide  of  mercury  formed ;  this,  however,  is  the 
same  as  happens  with  gum  solutions. 
Notwithstanding  the  numerous  instances  that  have  been  quoted,  the 
