ESTIMATION  OF  COMMERCIAL  SALTPETRES. 
543 
may  probably  be  referred  to  the  use  of  an  insufficient  amount  of 
this  agent — i.  e.,  where  the  wood  was  not  immersed  in  the  solu- 
tion for  a  sufficient  length  of  time.  The  action  should  be  one 
of  lixiviation,  not  merely  of  absorption. — Sillimans  Journal^ 
from  Programm  der  Realschule  za  Leipzig,  1861  ;  Boettgers 
polyt.  Notizhlatt, 
ON  THE  ESTIMATION  OF  COMMERCIAL  SALTPETRES, 
By  M.  J.  Persoz. 
The  best  method  for  estimating  the  water  is  to  melt  carefully 
from  50  to  200  grammes  of  nitre  in  a  platinum  capsule,  taking 
care  that  the  temperature  is  not  raised  much  above  the  fusing 
point.  Weigh  the  crucible  immediately  after  cooling.  If  the 
saltpetre  contains  nitrate  of  lime  or  magnesia,  add  about  1 
gramme  of  well-dried  neutral  chromate  of  potash,  which  pre- 
vents loss  of  nitric  acid  from  these  nitrates,  so  easily  decompos- 
ed by  heat. 
To  estimate  insoluble  matters,  treat  the  melted  mass  with 
water,  so  as  to  form  a  determinate  volume  of  solution  N.  Filter 
this  carefully ;  collect,  wash,  and  weigh  the  insoluble  precipi- 
tate. 
Gay-Lussac's  method  employed  for  the  estimation  of  chlorides 
is  to  use  two  standard  solutions,  one  containing  27  grammes 
and  the  other  2-7  grammes  of  pure  silver  to  the  litre.  Each 
cubic  centimetre  of  the  stronger  solution  corresponds  to  0-01466 
grammes  of  chloride  of  sodium  or  0-01864  grammes  of  chloride 
of  potassium. 
To  estimate  sulphates,  a  standard  of  chloride  of  barium  is 
used,  containing  259-8  grammes  of  salt,  each  cubic  centimetre 
representing  0-179  grammes  of  sulphate  of  soda,  or  0-208  gram- 
mes of  sulphate  of  potassa.  The  operation  is  conducted  in  the 
following  manner  : — Measure  200  cubic  centimetres  of  the  solu- 
tion N  of  nitre  ;  after  adding  a  few  drops  of  acid,  introduce  it 
into  a  platinum  capsule  and  boil  it  freely  ;  then  carefully  pour 
