Dye  a  skein  of  cotton  yarn  in  the  usual  manner  with  3  per  cent,
of  Diamine  Scarlet  36;  wash  well  (277),  and  top  as  before  with
J  per  cent,  of  Rhodamine  (278).
In  each  case  preserve  a  sample  of  the  skein  dyed  with  the
substantive  color  alone  and  compare  it  with  the  topped  sample.
NOTES.
i.  The  Substantive  Dyestuffs.  —  These  coloring-matters  were
first  discovered  in  1884  in  the  dyestuff  known  as  Congo  Red  by
Boettiger.  They  are  distinguished  by  the  common  property  of
dyeing  the  vegetable  fibres  in  full  and  comparatively  fast  shades
without  the  intervention  of  mordants;  they  also  dye  the  animal
fibres,  wool  and  silk.  Their  chief  application,  however,  is  to  cot-
ton. At  present  there  are  several  distinct  classes  of  substantive
dyes  as  far  as  their  chemical  constitution  is  concerned,  but  for
the  most  part  they  are  derived  more  or  less  directly  from  the  parent
substance  benzidine,  and  are  characterized  by  being  "tetrazo"
compounds;  that  is  to  say,  their  molecule  contains  the  azo  group
N  =  N  twice.  As  benzidine  is  a  diamine  compound  (that  is,
contains  the  diamine  group,  NH2,  twice),  these  colors  are  also
knowrr  as  the  "dianaine"  colors.  The  nature  of  the  dyeing
process  with  regard  to  the  substantive  colors  on  cotton  is  not  as
yet  thoroughly  understood ;  unlike  the  dyeing  of  the  acid  and  basic
colors,  there  appears  to  be  no  reason  for  assuming  that  a  chemi-
cal reaction  occurs  between  the  fibre  and  the  dyestuff.  The
substantive  dyes  as  a  rule  are  very  soluble  in  water,  and  conse-


154  DYEING  AND    TEXTILE  CHEMISTRY.
quently  the  dye-baths  are  seldom  completely  exhausted  even
when  relatively  small  amounts  of  the  coloring-matter  are  used.
