THE  TROPICAL  AGRICULTURIST. 
[Dec.  i,  1892" 
41S 
ozone  in  Nature,  and  not  artificially  prepared  ozone 
that  is  of  importance  in  the  bleaching  process. 
But,  how  does  ozone  act  as  a bleaching  agent  ? 
Let  me  first  state  that  ozone  bleaches  by 
oxidation.  Now  change  of  colour  is  a very  common 
accompaniment  of  chemical  change.  This  is  of 
the  utmost  importance  in  chemical  tests  as  all 
students  of  practical  chemistry  will  know — 
the  mere  application  of  heat  is  sometimes  suffi- 
cient to  produce  a loss  of  colour,  instance  the 
case  of  sulphate  of  copper  or  bluestone.  In  the 
same  manner  oxidation  nearly  always  produces 
change  of  colour  (as  in  the  conversion  of 
ferrous  into  ferric  oxide),  and  so  the  oxidation 
by  ozone  of  the  colouring  matter  (that  is  the 
coloured  substance  diffused  throughout  the  tissues 
of  the  material  to  be  bleached),  tends  to  produce 
a change  or  rather  a loss  of  colour  and  to 
whiten  the  material.  Why,  it  may  be  asked,  is 
it  that  the  oxygen  of  the  air,  constituting  as 
it  does  one-fifth  of  the  whole  atmosphere— why  is  it 
that  this  oxygen  does  not  readily  oxidise  all 
oxidisible  substances?  Well,  oxygen  does  oxidise 
a good  many  substances,  as  those  who  have 
studied  the  “ weathering  ” of  rocks  know  ; but  there 
are  a good  many  other  substances  which  are  not 
easily  or  not  at  all  oxidisable  by  the  normal  form 
of  oxygen,  but  require  the  more  active  form  of 
“ ozonized  oxygen  ” or  ozone  before  oxidation,  i.e., 
combination  with  oxygen  can  take  place. 
Again,  the  question  may  suggest  itself,  why  is 
ozone  a more  active  oxidiser  than  ordinary  oxygen  ? 
The  answer  to  this  question  is : In  consequence 
of  the  readiness  with  which  the  molecule  of 
ozone  gives  away  a part  of  its  oxygen  and 
reverts  to  the  normal  molecule  of  oxygen.  For 
reasons,  which  I will  not  attempt  to  explain 
here,  there  are  some  substances  which  are  stable 
and  others  which  are  unstable  ; that  is  to  say, 
some  have  a character  of  permanency  and  others 
have  a tendency  to  decompose  or  change  their 
Chemical  condition.  Ozone  is  one  of  these  un- 
stable bodies,  and  as  I said  before,  has  a tendency 
to  revert  to  the  normal  and  stable  condition 
of  oxygen,  whenever  circumstances  favour  the 
change.  This  may  be  better  understood  by 
saying  that  the  3rd  atom  of  oxygen  in  ozone 
or  03  is  not  firmly  held  in  the  molecule,  and 
has  therefore  a tendency  to  break  away  and 
join  some  other  substance  and  to  oxidize  it, 
leaving  behind  two  atoms  of  oxygen  which  form 
the  normal  oxygen  molecule.  (I  am  going  into  this 
explanation  rather  fully  since  it  applies  also  to 
the  case  of  other  bleaching  agents.) 
A molecule  is  the  smallest  particle  of  any 
substance  that  can  exist  in  a free  state,  and  a 
molecule  is  made  up  of  more  than  one  atom ; 
for  instance,  the  molecules  of  Hydrogen,  Nitrogen, 
and  Oxygen  each  consist  of  two  atoms,  and  they 
are  therefore  represented  by  the  molecular 
formula  Ha,  N2  and  O2  — the  figures  indicating 
the  number  of  atoms  in  the  molecule.  Ozone, 
as  I said,  consists  of  three  atoms  of  oxygen,  and  is 
represented  by  the  formula  03-  Now  when 
ozone,  for  the  reasons  mentioned,  tends  to  be 
reduced  to  02  (normal  oxygen^  one  atom  of 
oxygen  or  O becomes  separated>  but  as  we 
have  seen,  one  atom  of  oxygen  cannot  exist  in  a 
free  state  but  must  become  united  either  to 
another  atom  of  oxygen  or  to  another  atom 
of  some  other  element  or  to  a group  of  other 
dissimilar  atoms.  And  it  is  this  readiness  of 
the  atom  of  oxygen  that  lias  been  liberated  by 
ozone  on  its  conversion  to  common  oxygen,  it  is 
this  readiness  to  combine  with  other  substances 
that  makes  ozone  such  an  active  oxidising 
#gent,  that  is  suoh  a powerful  bleacU^S  agent, 
for  when  ozone  oxidises  a vegetable  colour  it 
bleaches  it.  The  condition  of  an  element  the 
moment  it  is  liberated  from  a compound  is  known 
as  the  “nascent  state,”  (which  is  in  reality 
only  a transient  state),  and  it  is  the  fact  that 
ozone  sets  free  “ nascent  oxygen”  that  makes  it  a 
more  active  chemical  agent  than  ordinary  oxygen, 
and  stamps  its  character  as  a powerful  oxidizer 
and  bleacher.  The  natural  process  of  bleaching  is 
also  taken  advantage  of  for  the  decolourizing  of  oils. 
Another  chemical  agent  that  bleaches  by  oxida- 
tion is  Hydrogen  peroxide.  The  molec.  formula 
of  this  substance  is  H2  O2,  and  it  may  be  said 
to  be  water,  which  is  H2  02,  with  an  additional 
atom  of  oxygen,  hence  it  has  been  called 
“ oxygenated  water.” 
Hydrogen  peroxide  was  discovered  by  M. 
Thenard  in  1818,  and  may  be  prepared  among 
other  ways  by  the  action  of  Hydrochloric  acid 
upon  Barium  dioxide : when  pure  H^Ch  is  a 
tbickish  liquid.  In  its  chemical  properties  it 
closely  resembles  ozone : and  as  ozone  tends  to 
give  up  one  atom  of  oxygen  and  revert  to 
common  oxygen,  so  H2O2  tends  also  to  give 
up  one  atom  of  oxygen  and  become  water.  Here 
again  it  is  the  nascent  oxygen  that  is  the  active 
agent  which  oxidises  and  bleaches  vegetable 
colouring  matters.  In  the  arts  peroxide  of 
oxygen  is  used  to  restore  the  lights  of  paintings 
that  have  become  darkened  by  sulphuretted 
hydrogen.  There  is  another  curious  use  to  which 
it  is  put.  Though  it  is  no  secret  that  the  art 
of  dyeing  is  made  use  of  by  some  vain  people, 
it  is  not,  I think,  commonly  known  that  they 
sometimes  avail  themselves  of  the  opposite  art 
of  bleaching  1 A weak  solution  of  peroxide  of 
hydrogen  is  sold  by  hair-dressers  for  transforming 
the  darker  shades  of  golden  tresses  into  the 
lighter  flaxen  tints. 
The  third  and  most  important  bleachiug  agent 
is  chlorine,  as  such  or  in  the  form  of  “ bleaching 
powder.”  It  is  the  chlorine  present  in  bleaching 
powder  which  is  the  active  agent  in  bleaching, 
and  if  we  understand  the  manner  in  which 
chlorine  acts,  we  shall  be  in  a position  to 
follow  the  bleaching  action  of  its  compounds. 
The  element  chlorine  which  is  a gas  was 
discovered  by  Scheele  in  1774.  It  is  not  found  free 
in  Nature,  but  many  of  its  compounds  occur — the 
most  familiar  being  sodium  chloride,  common 
salt.  Chlorine  can  be  prepared  from  common 
salt  by  beating  it  with  sulphuric  acid  or  oil 
of  vitriol,  but  is  generally  prepared  in  the 
laboratory  by  the  action  of  Hydrochloric  acid 
(also  called  muriatic  acid  or  spirits  of  salt)  on 
a black  powdery  substance  called  manganese 
dioxide.  Chlorine  is  a greenish  yellow  gas, 
whence  its  name,  possessing  a disagreeable  and 
peculiar  smell.  The  gas  when  present  in  any 
quantity  causes  violent  irritation  and  excessive 
coughing.  The  most  remarkable  property  of 
chlorine  is  its  affinity  for  hydrogen  with  which 
it  combines  to  form  hydrochloric  acid.  Now 
water  as  before  stated  is  chemically  represented 
as  H2O,  and  the  bleaching  action  of  chlorine 
depends  upon  its  power  of  combining  with  the 
hydrogen  in  water  and  liberating  the  oxygen. 
Chlorine  without  water  cannot  bleach,  and  water 
without  chlorine  cannot  bleach,  but  the  two  to- 
gether, forming  what  is  known  as  “wet-chlorine” 
exerts  a bleaching  action. 
Here  we  again  have  nascent  oxygen  whose 
powers  of  oxidation  and  bleaching  we  are 
already  acquainted  with.  Chlorine  may  be  termed 
an  “ indirect  bleacher,”  because  of  itself  it  cannot 
bleach — it  cannot  oxidise  because  it  has  no 
oxygen  to  give— but  it  is  able  by  its  attraction 
