88 
rochleder's  proximate  analysis. 
remains  no  other  means  for  the  separation  of  the  fat  than  to  precipitate  a 
portion  of  the  constituents,  which  are  dissolved  in  the  fatty  liquid  by  a  re- 
agent, and  then  to  filter  the  liquid.  It  is  evident  that  by  this  method  of 
separating  the  fiit  a  portion  of  the  other  constituents  are  sacrificed.  It  is 
advisable  to  avoid  such  loss  when  it  can  be  prevented. 
Section  2. —  On  the  nature  of  the  substance  to  he  examined,  and  on  the 
quantity  required  for  its  investigation. 
If  vs^e  desire  the  investigation  of  one,  several,  or  all  the  constituents  of  a 
plant,  by  means  of  analysis,  we  should  endeavor  to  obtain  the  material 
for  the  investigation  in  the  freshest  condition  possible.  I  have  had  oc- 
casion to  convince  myself  that  some  fresh  vegetable  substances,  even  when 
most  carefully  dried  and  preserved,  did  not,  after  some  months,  contain 
even  a  trace  of  several  bodies  which  were  easily  detected  in  their  recent 
state.  It  is  often  impossible  to  use  the  material  in  a  fresh  state  for 
investigation.  When  plants  or  their  constituents  have  to  be  examined, 
which  are  brought  from  distant  countries,  we  must  be  contented  to  obtain 
them  in  the  best  possible  state  of  preservation,  as  it  is  not  possible  to 
draw  with  full  certainty  a  correct  conclusion  from  the  composition  of  such 
dried  materials  as  to  their  composition  when  in  a  fresh  condition.  Indeed, 
such  investigations  are  calculated  to  afi"ord  very  useful  results  in 
pharmacological,  dietetic,  or  industrial  relations,  because  the  materials  are 
employed  in  the  same  condition  in  which  they  are  investigated  ;  but  they 
are  not  suitable  for  giving  a  representation  of  the  true  composition  of  the 
living  plant.  Chemical  changes  often  proceed  on  keepiDg  the  plants  with- 
out observable  external  alterations,  and  the  composition  resulting  from 
these  changes  is  then  often  regarded  as  the  original  one.  Consequently, 
when  we  are  in  a  position  to|  select  the  material  for  the  analysis  of  a 
plant,  and  are  not  compelled  to  employ  for  the  investigation  vegetable  sub- 
stances which  we  cannot  procure  in  a  fresh  state,  it  is  always  most  con- 
venient  to  select  such  plants  or  their  parts  as  can  be  obtained  in  a 
fresh  conditioD,  which  enable  us  to  undertake  the  investigation  at  places 
not  far  distant  from  where  they  grow.  Only  in  these  cases  are  we  sure  to 
obtain  a  correct  result  from  a  correct  analysis  ;  that  is,  to  be  able  to  learn 
the  true  composition  of  the  living  plant.  Results  are  only  obtained  in  this 
w^ay,  which,  in  vegetable  physiology  or  botany,  are  truly  trustworthy.  It 
would  appear,  from  what  has  been  stated,  that  the  field  of  activity  is  a  cir- 
cumscribed one  ;  but  this  is  not  so.  We  know  the  composition  of  plants 
which  grow  in  our  immediate  neighborhood,  less  than  that  of  many  others. 
The  tree  under  whose  shadow  we  walk,  as  well  as  the  vegetable  we  tread 
upon  in  our  path,  are  chemically  unknown  things.  We  know  the  salicin 
of  the  willow  and  the  populin  of  the  poplar,  we  know  the  amygdalin  of 
the  almond  and  the  volatile  oils  of  the  chamomile  and  the  sage,  but  a  repre- 
sentation of  the  composition  of  those  plants  we  have  not.  He  who  knows 
the  composition  of  oil  of  valerian,  knows  nothing  of  the  composition  of  the 
(To  be  contiuued.) 
