38B 
treatment  of  freshly  coaorulated  and  pressed  “biscuits”  of  rubber. 
Anything  in  the  way  of  heating  to  accelerate  the  drying  of  the 
biscuits  is  to  be  avoided  and  increased  rapidity  must  be  ob- 
tained by  the  employment  of  currents  of  dry  air  passing  over  the 
biscuits  which  must  also  be  kept  as  thin  as  possible  if  rapid  drying 
is  desired.  On  heating  India  rubber  to  a greater  extent,  it  melts 
and  then  begin  to  decompose,  breaking  down  into  various  liquid 
products  which  can  be  distilled  off. 
On  heating  with  free  exposure  to  air  the  rubber  takes  fire  and 
burns  freely.  It  is  not  commonly  realised  how  inflammable  India 
rubber  is,  and  to' obtain  real  conviction  of  this  property  a small 
strip  of  dry  rubber  should  be  burnt. 
With  the  majority  of  chemical  r.e-agents  India  rubber  shews  no 
action,  this  being  a property  which  it  holds  in  common  with  many 
other  hydro-carbons.  It  does,  however,  re-act  with  the  strong 
mineral  acids,  and  with  chlorine  and  bromine.  Its  re-action  with 
sulphur  is  the  most  important,  if  heated  with  this  substance  (or 
if  a solution  of  rubber  be  treated  with  sulphur  monochloride)  the 
rubber  combines  with  the  sulphur  and  becomes  both  chemically 
and  physically  more  inert  than  before.  This  process  of  combina- 
tion with  sulphur  is  called  vulcanisation  and  after  vulcanisation 
the  rubber  is  no  longer  soluble  in  the  solvents  mentioned,  nor, 
which  is  more  important,  does  it  become  sticky  on  gentle  heating, 
nor  brittle  when  exposed  to  cold.  In  fact  if  it  were  not  for  this 
capacity  India  rubber  possesses  for  absorption  of,  and  combination 
with  sulphur  and  consequent  change  of  properties,  the  India  rubber 
industry  could  scarcely  have  developed  i\t  all. 
The  amount  of  sulphur  which  can  be  combined  with  the  rubber 
in  the  process  of  vulcanisation  varies  from  a lower  limit  of  about 
3 per  cent,  to  an  upper  limit  of  32  per  cent.,  and  the  product 
becomes  harder  and  darker  as  the  amount  of  sulphur  is  increased 
until  hard  rubber,  that  is  vulcanite  and  ebonite,  results  when  20 
per  cent,  of  sulphur  has  been  combined  w ith  the  rubber.  Between 
these  limits  there  is  every  gradation  of  product  possible. 
Towards  oxygen  and  oxidising  agents  generally,  rubber  is  com- 
paratively inert,  but  with  the  oxygen  of  atmospheric  air  there  is 
some  slight  action  resulting  in  the  formation  of  a resinous  oxida- 
tion product.  As  might  be  expected  this  action  in  its  rapidity 
depends  upon  the  porosity  and  condition  of  the  rubber  with  respect 
to  the  amount  of  surface  exposed:  with  crude  rubber,  as  prepared 
from  the  latex,  this  action  in  a period  of  six  months  ap^pears  to  be 
quite  negligable,  and  little  fear  of  deterioration  of  the  rubber 
during  its  transport  to  Europe  need  be  apprehended. 
I he  foregoing  account  of  the  properties  of  rubber  is  extremely 
brief,  fuller  details  and  a further  account  of  many  points  not  even 
touched  upon,  can  be  found  in  Weber’s  Chemistry  of  India  Rubber. 
In  my  next  letter  I propose  to  consider  the  other  constituents  of 
the  latex  and  then  shall  deal  with  practical  methods  of  preparation 
