A  janT^yPi905m }  Expert  Testimony.  g 
hand,  when  a  fact  of  this  nature  is  uncommon  it  is  perfectly 
proper  to  bring  out  its  rarity ;  and,  if  such  be  the  case,  to  show  that 
while  an  integral  portion  of  the  witness's  knowledge,  it  is,  neverthe- 
less outside  of  his  personal  observation. 
So  much  for  the  one  position.  Now  for  the  other.  To  confine 
ourselves  to  a  chemical  illustration,  which  may  have  toxicologic 
bearing,  let  us  take  Gautier's  observation  of  the  presence  of  arsenic 
in  the  normal  human  body.  If  this  bare  statement  should  be  per- 
mitted to  be  made  in  court  in  a  case  in  which  the  presence  of 
arsenic  in  a  given  body  is  the  basis  of  a  murder  trial,  one  can  see 
that  justice  might  easily  be  defeated.  Were  Gautier  himself  testi- 
fying, he  would  say,  as  he  has  written,  that  arsenic  could  be  found 
in  normal  human  bodies  only  by  the  use  of  a  certain  very  delicate 
technique,  which  I  need  not  here  describe  ;  that  he  had  found  it 
only  in  the  ectodermic  structures,  and  chiefly  in  the  thyroid  gland  ; 
that  he  had  never  found  it  in  the  normal  liver  or  normal  stomach  or 
normal  muscles.  He  would  add,  that  even  in  the  thyroid  gland 
he  had  found  it  only  in  minute  quantity — about  y1^-  of  a  milli- 
gramme in  the  average  human  thyroid ;  that  is  to  say,  in  the 
proportion  of  about  400000000  °^  tne  average  body  weight ;  so 
that  if  by  any  inconceivable  post-mortem  diffusion  the  thyroid' 
arsenic  should  escape  into  all  the  tissues,  it  wojjld  be  in  a  propor- 
tion absolutely  beyond  detection — the  least  that  he  has  been  able  to 
detect  being  ?0  0  010  0  0  0>  that  is  to  say,  of  a  milligramme  in  100 
grammes  of  tissue.  To  translate  this  into  grains,  it  would  take  1 00 
thyroids — or,  if  diffused,  1 00  entire  human  bodies — to  yield 
%  grain  of  normal  arsenic  to  Gautier's  test ;  or  to  follow  Gautier's 
fair  statement  to  the  letter,  even  doubling  the  assumed  quantity  of 
normal  arsenic  in  a  cadaver  to  -^fo  of  a  milligramme  to  allow  for 
possible  traces  in  the  skin  and  its  appendages,  we  should  need  a 
mass  equal  to  fifty  bodies  to  give  us  the  yl  grain.  A  simple  arith- 
metic calculation  therefore  shows  that  this  observation,  whatever 
may  be  its  physiologic  importance,  has  little  medico-legal  bearing. 
But  the  partial  expert  witness — that  is,  one  who  is  telling  only  part  of 
the  truth  in  order  to  serve  one  side  of  the  case — might  say  that 
Gautier  had  found  arsenic  in  the  normal  human  body  in  recogniz- 
able quantity  and  omit  to  say  anything  further.  Or  he  might  be 
ignorant  of  the  details  of  Gautier's  studies,  even  were  the  attorney 
for  the  prosecution  sufficiently  well  posted  to  inquire  concerning 
