Vegetable  Chemistry. 
383 
axiom  of  Liebig’s: — “The  crops  in  a field  diminish  or  increase 
in  exact  proportion  to  the  diminution  or  increase  of  the  mineral 
substances  conveyed  to  them  in  manure.”  This  doctrine  received 
a deathblow  from  Mr.  Lawes’s  experiments  at  Rothamstead  in  the 
following  manner : — Plants,  it  is  well  known,  consist  chiefly  of 
vegetable  matter  formed  of  oxygen,  hydrogen,  nitrogen,  and 
carbon;  the  first  gas  forming  water  with  the  second,  air  with  the 
third,  and  with  the  fourth  substance  carbonic  acid,  a gas  largely 
diffused  in  the  atmosphere.  These  then  are  the  organic  elements 
of  plants.  But  further,  the  ashes  of  plants  contain  various  mine- 
rals, potash,  soda,  lime,  &c.,  combined  with  phosphoric  and  sul- 
phuric acids  in  various  quantities,  and  it  was  with  these  last,  the  mi- 
neral materials,  that  we  were  taught  to  build  up  our  crops.  Those 
indeed  who  were  indisposed  to  extreme  theories  saw  that  two 
chemical  bodies  singled  themselves  out  from  all  others  by  efficacy, 
namely,  Phosphorus  and  Ammonia : the  former  as  super-phos- 
phate, seemed  alone  sufficient  for  turnips,  but  was  also  applied 
singly  to  wheat ; the  salts  of  ammonia,  an  organic  manure  as  acting 
by  its  nitrogen,  were  applied  not  only  to  corn,  but  to  roots  also.  It 
is  necessary  thus  to  recall  past  uncertainty  that  we  may  do  justice 
to  Mr.  Lawes  and  Dr.  Gilbert.  Their  experiments  appeared  in 
this  Journal  ;*  and  the  shortest  extract  only  need  be  given  here. 
Wheat  was  grown  at  Rothamstead  on  an  arable  field  exhausted 
for  the  purpose,  and  the  results  were  as  follows  : — 
Bushels  of  Wheat  per  Acre. 
Unmanured  ...  ...  16f 
700  lbs.  superphosphate  ....  16J 
Eight  lots  with  various  phosphates.  Average  16| 
Ash  of  14  tons  of  farm-yard  dung  ...  16 
14  tons  of  farm-yard  dung  ....  22 
It  is  evident  that  on  this  exhausted  land  the  heavy  dose  of  super- 
phosphate and  the  other  mineral  dressings  did  nothing  at  all. 
They  might  have  been  ill  selected,  but  the  mineral  contents  of 
the  dung  did  nothing ; and  this  should  be  carefully  observed,  for 
the  mineral  theory  lingers  still  among  us,  and  it  has  been  stated 
quite  recently,  in  a popular  lecture,  that  the  slight  mineral 
contents  of  a cartload  of  dung  are  all  that  is  wanted  by  wheat. 
The  effect,  however,  was  clearly  in  the  organic  contents  of  the 
dung.  This  working  power  might  be  either  in  the  carbon  (char- 
coal or  woody  matter)  or  the  nitrogen f (ammonia)  of  the  dung. 
Unfortunately  Mr.  Lawes  did  not  use  ammonia  singly,  but  he  used 
it  in  combination  with  the  following  minerals,  which  we  have  seen 
to  be  valueless  when  standing  alone : — 
* Lawes  on  Agricultural  Chemistry,  Journal,  viii.  226,  and  viii.  494. 
f Ammonia  consists  of  nitrogen  and  hydrogen.  The  former  constitutes  its  value. 
Where  no  precision  of  scientific  language  is  necessary,  it  is  sometimes  more  convenient 
to  use  the  term  nitrogen,  and  sometimes  the  term  ammonia. 
2 c 2 
