ON  THE  ADULTERATION  OF  CARMINE. 
361 
cles  they  swell  up  when  moist  and  become  easily  penetrated  by 
a  liquid  throughout  their  entire  mass ;  on  the  other  hand  the 
cell-walls  of  cotton  fibres  are  denser,  less  penetrable  and  at  the 
same  time  thinner  and  therefore  unable  to  contain  the  same 
quantity  of  liquid. 
It  has  been  often  urged  that  since  fibres,  especially  those  of 
animal  origin,  not  only  exert  an  attraction  for  salts,  &c,  but  also 
possess  the  power  of  decomposing  some  of  them,  their  action 
must  be  chemical.  But  in  this  respect  the  behaviour  of  charcoal 
is  similar  to  that  of  the  fibres.  So  too  with  regard  to  the  in- 
creased attraction  for  color  exhibited  by  mordanted  cotton  which 
is  on  a  par  with  the  fact  observed  by  Stenhouse  that  the  decol- 
orizing power  of  wood  charcoal  is  considerably  increased  by  pre- 
cipitating alumina  upon  it. 
According  to  the  Author,  mordants  act  by  producing  insoluble 
colors  (lakes.)  Their  behaviour  towards  coloring  matters  in  solu- 
tion must  be  ascribed  to  chemical  affinity,  with  which  however 
the  fibres  themselves  have  nothing  to  do. 
The  so-called  substantive  dyes  become  insoluble  from  some 
other  cause  than  the  addition  of  a  mordant;  for  example,  oxyda- 
tion  of  protoxyd  of  iron,  or  of  white  indigo. 
That  common  alum,  with  which  wood  or  silk  has  been  impreg- 
nated, is  able  to  attract  coloring  matter  from  solutions  and  pre- 
cipitate it  on  the  fibres  depends  not  upon  the  strength  of  the 
chemical  affinity  of  these  fibres  for  the  coloring  matter,  but  upon 
the  fact  (experimentally  proved  by  Bolley)  that  they  become 
saturated  with  the  alum,  which  cotton  does  not. — Am.  Journ. 
Science  and  Arts,  May,  1860. 
ON  THE  ADULTERATION  OF  CARMINE. 
By  J.  Attfield, 
Demonstrator  of  Chemistry  at  St.  Bartholomew's  Hospital. 
The  coloring  matter  of  the  cochineal  insect  has  been  shown  by 
Dr.  De  la  Rue  to  be  a  definite,  though  scarcely  crystalline, 
proximate  principle ;  and  from  its  source  and  properties  has  had 
the  name  carminic  acid  given  to  it.  This  acid  united  with  about 
five  per  cent,  of  alumina,  or  sometimes  with  oxide  of  tin,  and  oc- 
