416  The  "Drop"  Method  of  Chemical  Analysis.    {^^gg  ™8a4rm- 
been  called.  The  author  explains  these  by  the  formation  of  a  double 
salt  and  the  consequent  separation  of  the  alkaline  salt  along  with  the 
haloid  salt  in  the  solid  form.  The  displacement  of  hydrochloric  acid 
united  to  silver,  by  hydrobromic  and  hydriodic  acids,  whether  free  or 
combined  with  alkalis,  takes  place  by  reason  of  the  thermic  pre- 
ponderance of  these  last  acids,  and  would  exclusively  take  place  but 
for  the  existence  of  double  and  acid  salts  capable  of  giving  rise  to  in- 
verse reactions,  which  however  are  limited  by  the  dissociation  of  the 
double  salts.  And  with  the  silver  salts,  as  with  the  mercuric,  the 
whole  resolves  itself  into  one  fundamental  action,  namely,  that  which 
corresponds  with  the  thermic  maximum. — Jour.  Chem.  Soe.,  1884,  p. 
656;  Ann.  Chim.  Phys.,  [5],  29,  p.  198-288. 
THE  "DROP"  METHOD  OF  CHEMICAL  ANALYSIS. 
By  Dr.  H  Hagek. 
The  customary  methods  for  testing  medicinal  agents,  which  are  both 
tedious  and  require  a  larger  quantity  of  material,  can  be  superseded 
bv  a  method  which  requires  merely  single  drops  of  the  reagent,  as  well 
as  of  the  liquid  to  be  examined.  For  this  method  the  following  re- 
agents are  needed : — 
Red  and  blue  litmus  paper  and  turmeric  paper. 
Extract  of  indigo  paper,  which  is  turned  yellow  by  hot  nitric  acid 
and  caustic  alkalies,  but  not  by  ammonia. 
Rosaniline  paper  as  a  test  for  alcohol. 
Potassium  ferrocyanide  paper  as  a  reagent  for  ferric  salts  (blue), 
copper  and  uranium  (deep  brown),  gold  (greenish  brown),  platinum 
(brownish  green  to  reddish),  thallium  and  vanadic  acid  (yellow). 
Potassium  sulphocyanide  paper  is  turned  decidedly  yellow  by 
bismuth  nitrate,  bluish  black  by  salts  of  copper,  red  by  solution  of  gold, 
white  by  mercuric  nitrate,  black  by  mercurous  nitrate,  and  blood-red 
by  ferric  salts. 
Potassium  iodide  paper  is  turned  red  by  mercuric  salts,  green  by 
mercurous  salts,  yellow  by  solution  of  lead.  For  detecting  chlorates 
2  to  3  cc.  of  the  liquid  are  placed  in  a  small  test-tube  along  with  a 
slip  of  the  paper ;  1  cc.  of  dilute  sulphuric  acid  is  then  added,  and 
heat  is  applied.    If  chlorate  is  present  the  liquid  turns  yellow. 
Mercurous  nitrate  paper  serves  when  moistened  to  detect  ammonical 
