SPIRITUS  iETHERIS  NITRICT. 
301 
mixture  and  its  two  elements  (the  small  portion  of  water  and 
aldehyd,  and  degrees  of  temperature  within  five  or  six  being  dis- 
regarded,) from  which  to  calculate  the  proportion  of  each  ele- 
ment, by  the  following  rule  : 
"  Take  the  difference  of  every  pair  of  the  three  specific  gravi- 
ties, viz.,  of  the  compound  and  each  ingredient ;  and  multiply  the 
difference  of  every  two  by  the  third.  Then  as  the  greatest  pro- 
duct is  to  the  whole  weight"  or  volume  "  of  the  compound,  so 
is  each  of  the  other  products  to  the  weight"  or  volume  "  of  the 
two  ingredients."  The  specific  gravity  of  the  mixture  being 
above  or  below  the  mean  of  the  two  ingredients,  indicates  that 
the  largest  obtained  proportional  belongs  on  the  side  of  the 
greater  or  less  specific  gravity. 
The  descriptions  of  sweet  spirit  of  nitre  as  commonly  met  with 
are  faulty.    It  is  described  by  our  Pharmacopoeia  in  common  with 
others  as  »  colorless."    I  have  never  seen  it  colorless  when  of 
fair  quality,  no  matter  by  what  process  it  may  have  been  made. 
On  the  other  hand,  when  of  full  officinal  (U.  S.  P.)  strength,  it 
has  a  very  decided  greenish  yellow  tint.    That  it  may  be  made 
colorless  by  repeated  rectification  of  the  ether,  I  have  no  doubt ; 
neither  of  the  formula,  however,  accomplish  this,    It  is  said  that 
it  "slightly  redder s  litmus,"  but  when  freshly  made,  or  when 
well  kept,  it  does  not  at  all  redden  ordinary  litmus  paper  until  a 
few  moment's  exposure  to  the  air,  on  the  paper,  oxidizes  the 
aldehyd  or  binoxide  of  nitrogen  which  it  contains,  in  small  pro- 
portion.   This  change  occurs  the  more  quickly,  and  is  the  more 
marked,  as  the  preparation  contains  more  water.    The  strictly 
officinal  preparation  begins  to  boil  at  156°  to  157°  instead  of 
u  160°."    It  is  described  as  having  a  s.  g.  of  .834,  a  density  be- 
low that  of  the  alcohol  used,  whilst  the  ether  dissolved  in  the 
alcohol  has  a  density  of  .947.    In  practice  the  s.  g.  varies  from 
.840  to  .841  when  freshly  made.    It,  however,  loses  in  s.  g.  and 
in  color,  by  age,  without  giving  evidence  of  any  other  change. 
Several  specimens  put  up  at  .840  to  .842  were  examined  at  periods 
varying  between  two  years  and  five  months,  were  found  to  have 
s.  g.  of  .835  to  .838,  yet  were  not  deficient  in  the  proportion  of 
ether  yielded  to  fractional  distillation. 
Tests  of  character  for  this  preparation  are  much  needed,  but 
it  is  not  easy  to  find  any  that  are  simple  and  reliable.    The  least 
