2.  Persons  who,  although  not  practical  artisans,  are  thoroughly
conversant  with  the  subject  of  mechanics  as  a  science,  as  to  the
questions  of  novelty  of  invention,  or  identity  or  divei'sity  of
mechanical  apparatus,  and  contrivances,  and  equivalents.  Allen  v..
Blunt,  3  Story,  742,  lil  ;  8  Z.  Rep'r,  165.  S.  P.,  Olcott  w.  Hawk-
ins, 2  Am.  L.  J.  N.  S.  321.
The  testimony  of  a  chemist,  who  has  analyzed  the  ingredients
of  a  composition  of  matter,  as  to  the  nature  of  such  composition,  is
not  matter  of  opinion,  but  evidence  of  a  fact  demonstrated.  Allen
V.  Hunter,  6  McLean,  303.
Where  the  opinions  of  experts  are  expressed  in  ex  parte  affida-
vits, and  there  is  no  opportunity  for  the  court  to  ascertain  in  what
sense  important  words  are  used,  nor  what  facts  they  take  in  view,
nor  what  standards  of  comparison  they  assume,  their  opinions  are
of  very  little  value.  Sargent  v.  Carter,  1  Fish.  Pat.  Cas.  211  ;  11
i.  JRep'r,  JV.  S.  651.
In  a  suit  for  infringement  the  testimony  of  a  mechanical  expert
is  to  be  tried  by  the  tests  applied  to  the  evidence  of  other  wit-
nesses.    May  V.  County  of  Fond  du  Lac,  27  I^ld.  Rep'r,  691.
Expert  testimony  is  not  essential  to  prove  an  alleged  infringe-
ment of  a  patent.  Hayes  v.  Bickelhoupt,  23  Fed.  RepW,  1S3  ;  32
Eat.  Off.  Gaz.  133.
Experts  assume  the  duty  of  the  court  when  they  undertake  to
tell  what  the  patent  is  for,  or  what  is  or  is  not  a  violation  of  the
patent.  Waterbury  Brass  Co.  v.  iSlew  York  &  Brooklyn  Brass  Co.,
3  Msh.  Pat.  Cas.  43.
294.  Evidence,  with  Reference  to  Particular  Pleadings.
