14 
PRACTICAL  KNOWLEDGE. 
and  will,  very  likely,  be  led  to  study  out  its  philosophical  princi- 
ples, the  best  methods  of  conducting  it,  &c. 
Having  now  obtained  the  necessary  quantity  of  the  iron  salt, 
he  proceeds  to  the  next  step  of  the  process — its  conversion  into 
the  hydrated  oxide.  He  is,  doubtless,  a  reader  of  the  American 
Journal  of  Pharmacy,  and  therefore  will  select,  as  being  the 
most  convenient,  the  process  of  Prof.  Procter  for  this  substance, 
given  in  the  March  number  of  that  Journal  for  1853.  Here 
again  transpires  interesting  chemical  changes,  prominent  among 
which  he  will  note  the  rapid  oxidation  of  the  sulphate  of  iron 
by  nitric  acid,  accompanied  with  the  evolution  of  red,  suffocating 
fumes.  In  studying  the  nature  of  these  changes  he  will,  in  order 
to  a  clear  understanding  thereof,  acquaint  himself  with  the 
chemical  properties  of  nitric  acid  and  incidentally  its  history, 
mode  of  manufacture,  &c.  He  will  also  have  observed,  during 
the  washing  of  the  oxide,  that  the  salt  of  baryta  continued  to 
give  unmistakable  evidence  of  the  presence  of  sulphate  of  am- 
monia long  after  the  tongue  had  ceased  to  reveal  it  to  the  taste. 
He  cannot  but  be  struck  with  the  delicacy  of  this  test,  and  per- 
haps, as  the  value  of  this  kind  of  knowledge  becomes  apparent 
to  him,  it  may  be  the  means  of  leading  him  to  the  systematic 
study  of  that  most  interesting  and  useful  branch  of  chemical 
science,  qualitative  analysis.  Another  fact  connected  with  this 
substance,  which  must  not  be  overlooked,  is  its  importance  as  an 
antidote  to  arsenic.  He  must  inform  himself  of  its  value  in 
this  respect,  its  modes  of  administration  and  the  best  method  of 
keeping  it  on  hand  for  cases  of  emergency.  The  thoroughly 
washed  and  drained  magma  is  now  added  to  a  definite  amount  of 
citric  acid  in  solution,  and  the  mixture  maintained  at  a  certain 
temperature  till  the  saturation  of  the  acid  is  complete.  Here 
the  reasons  for  this  certain  amount  of  citric  acid  and  this 
limited  range  of  temperature  may  be  inquired  into. 
Upon  investigation,  he  will  find  that  experience  has  proved  the 
solubility  of  the  oxide  to  be  greater  within  that  temperature 
than  any  other,  and  that  the  proper  proportions  of  the  ingredi- 
ents of  all  chemical  formulae  are  regulated  by  a  fixed  law,  viz  : 
the  law  of  equivalent  combination,  by  which  bodies  can  only 
unite  in  certain  invariable  proportions.  The  further  study  of 
this  subject  will  introduce  him  to  the  knowledge  of  the  atomic 
