264         YELLOW  AMORPHOUS  OXIDE  OF  MERCURY,  ETC. 
the  most  careful  possible  way,  is  submitted  to  the  microscope,  it 
may,  under  a  magnifying  power  of  even  120  diameters,  be  recog- 
nized as  a  mass  of  broken  crystals.  The  point  up  to  which  the 
trituration  should  be  continued,  which  forms  the  measure  of  the 
fineness  of  the  division,  is  in  this  method  uncertain  and  incon- 
stant. Thus  this  preparation  occurs  in  different  degrees  of  fine- 
ness in  different  shops  ;  and  as  its  efficacy  is  intimately  connected 
with  its  fineness,  the  surgeon  gets  preparations  which  act  with 
unequal  strength. 
For  obtaining,  therefore,  a  preparation  uniform  in  its  effects, 
and  in  the  finest  possible  state  of  division,  the  yellow  precipita- 
tate,  which  is  thrown  down,  is  highly  to  be  recommended. 
Thinking  this  would  also  prove  a  far  more  energetic  preparation, 
I,  in  1856  for  the  first  time,  prepared  some,  and  recommended 
its  use  to  Dr.  Pagenstecher  in  his  eye-practice,  instead  of  the 
common  precipitate,  and  found  my  anticipation  most  gratifyingly 
confirmed.  The  mode  of  preparing  the  yellow  precipitate, 
although  well  known,  may  be  still  worth  mentioning.  Care  must 
be  taken  in  the  precipitation  to  obtain  a  pure  oxide,  and  not  any 
of  its  compounds,  to  which  precipitates  of  mercury  have  a  great 
tendency — a  fact  which  might  detract  from  the  efficacy  of  the 
preparation.  The  precipitation  is  effected  by  adding  a  solution 
of  the  chloride  of  mercury  to  a  solution  of  potash,  in  such  a  way 
that  there  is  always  an  excess  of  the  latter.  After  the  precipi- 
tate has  deposited  itself,  the  supernatant  fluid  is  at  once  poured 
off,  the  precipitate  thoroughly  washed  with  distilled  water,  and 
dried  by  a  gentle  heat,  with  exclusion  of  daylight.  Thus  pre- 
pared, the  yellow  precipitate  has  a  light-yellow  (that  of  the  yolk 
of  egg)  color,  and  is  an  exceedingly  fine  powder,  which,  even 
under  the  microscope,  appears  completely  amorphous.  In  addi- 
tion to  both  the  above  signalized  properties,  it  differs  from  the 
ordinary  precipitate  in  its  chemical  behaviour,  being  much  more 
quickly  acted  on  by  reagents.  A  solution  of  oxalic  acid,  which 
acts  on  the  red  oxide  only  after  boiling,  very  quickly  changes 
the  yellow  oxide,  even  at  the  ordinary  temperature,  into  the 
white  oxalate.  The  preparation  of  hypochloric  acid  gas  depends 
on  the  property  the  yellow  oxide  of  mercury  possesses  of  decom- 
posing in  contact  with  chlorine  gas  ;  the  results  being  hypochloric 
