Conversely,  cause  being  given  to  find  its  effect,  if
in  several  instances  an  alkali  and  oil  {A)  unite,  e.  g.
potash  and  tallow,  soda  and  suet,  lime  and  olive  oil,
a  common  circumstance  is  soap  {z) ;  this,  then,  is  the
effect  of  the  common  antecedent.
From  each  of  these  particular  determinations  an
induction  is  now  competent,  thus :  Any  transparent
film  in  white  light  exhihits  jprismatio  colors;  and
Any  alkali  and  oil  uniting  produce  a  soap.
Some  other  examples  will  be  helpful.  We  observe
in  many  cases  the  conversion  of  solids  into  liquids,
and  these  into  gases.  The  bodies  so  converted  have
a  great  variety  of  properties.  One  circumstance
common  to  the  cases  is  the  increase  of  heat.  The
elimination  of  other  circumstances  being  complete,
this  antecedent  is  rightly  assigned  as  the  cause  of  the
change.
AGREEMENT  119

Brewster  proved  that  the  iridescence  of  nacre  is
not  due  to  the  nature  of  the  substance^,  but  of  the
surface.  Taking  an  impression  of  it  in  wax,  he  found
on  this  different  substance  a  like  iridescence.  It  is
now  a  familiar  fact  that  the  surface  of  glass  or  metal,
when  finely  grooved,  becomes  iridescent.
If  a  certain  occupation  or  mode  of  living  is  found
to  be  usually  attended  by  a  particular  disease,  it  is
reasonably  suspected  to  be  the  cause  of  the  disease ;
and  the  exceptional  cases  wherein  the  disease  does  not
occur  are  suspected  to  involve  a  preventive  cause
(§§15,47,  56  n.).
Whenever  I  eat  a  particular  kind  of  fruit,  what-
ever else  I  may  eat  or  drink,  however  various  my
general  state  of  health,  the  temperature  of  the  air,
the  season,  the  climate,  and  divers  other  surround-
ings, I  am  taken  ill,  and  rightly  consider  the  eaten
fruit  the  very  probable  cause.
