ANALYSIS  OP  PRUSSIC  ACID,  AC. 
131 
The  test  solution  which  I  have  referred  to,  is  simply  a  so- 
lution of  nitrate  of  silver,  made  by  dissolving  40  grains  of 
pure  fused  nitrate  in  2373  grains  of  distilled  water. 
Description  of  the  test  tube. — This  tube,  which  is  repre- 
sented in  the  figure,  is  about  half  an  inch  inside  diameter, 
and  12  inches  long,  is  closed  at  one  end  and  finished  at 
the  other  with  a  thin  edge  and  without  a  lip,  so  as  to  be 
securely  closed  with  the  thumb.  The  graduations  are 
made  by  weighing  off  distilled  water  in  successive  por- 
tions. Thus  to  the  lowest  mark  (a)  the  tube  contains 
20  grains.  To  the  next  line  above  (marked  on  the 
tube  J)  40  grains  more,  to  the  line  marked  J,  40  grains 
more,  and  to  the  line  marked  0,  being  the  commence- 
ment of  the  percentage  scale,  80  grains  more,  or  180 
grains  in  all.  The  measures  of  the  scale,  each  marked 
to  indicate  one-tenth  of  one  percent,  have  the  capacity 
of  30  grains  each,  and  are  subdivided  into  10  parts. 
All  the  above  marks  are  drawn  as  tangents  to  the 
lowest  point  of  curvature  of  the  liquid  in  the  tube  (see  a 
in  the  figure)  and  should  always  be  so  read  off. 
Process  of  Analysis. 
1st.  Drop  into  the  tube  about  half  a  grain  of  common  salt,* 
and  fill  up  to  the  lowest  mark  with  solution  of  caustic  potassa, 
(U.  S.  Pharm.)  allowing  the  solution  some  time  to  settle  down  to 
its  proper  level. 
2d.  Carefully  fill  up  to  the  line  0  with  the  liquid  to  be  tested, 
avoiding  the  slightest  excess. 
3d.  Drop  into  the  tube  the  test  solution,  shaking  after  each 
addition,  until  the  precipitate  which  appears,  ceases  to  be  re- 
dissolved,  and  the  liquid  shows  a  slight  cloudiness  remaining. 
4th.  The  surface  of  the  liquid  now  indicates  the  percentage, 
which  is  that  of  the  volume  of  the  liquid  ;  and  as  the  specific 
gravity  of  the  distilled  medicated  waters  do  not  differ  appreciably 
from  pure  water,  it  will  also  represent  the  percentage  by  weight. 
*  Chloride  of  sodium,  being  the  most  delicate  test  for  silver  is  added,  in 
order  to  detect  the  minutest  excess  above  that  necessary  to  form  the  double 
cyanide  of  silver  and  potassium. 
