A  janu^yPih4rm";    Presence  of  Arsenic  in  Chemicals.  31 
the  chemical — an  amount  which  certainly  might  cause  alarming 
symptoms  when  taken  ad  libitum. 
In  1775  Scheele1  made  the  important  discovery  that  arsenic  united 
with  hydrogen  to  form  a  foetid  gas  was  decomposed  by  heat. 
Proust2  observed  that  this  same  gas  was  disengaged  when  arsenical 
tin  was  dissolved  in  hydrochloric  acid,  and  arsenic  was  deposited 
when  the  inflamed  gas  was  brought  against  a  cold  surface.  Tromms- 
dorf3  next  announced  that  arsenical  hydrogen  was  evolved  when 
arsenical  tin  was  treated  with  dilute  sulphuric  acid,  and  if  this  gas 
was  passed  through  a  sufficiently  long  tube,  arsenic  was  deposited 
on  its  walls.  Arsenical  hydrogen  was  further  studied  by  Davy,  Gay- 
Lussac,  Gehlen,  Stromeyer,  Thenard ;  and  Serrullas  in  1821  pro- 
posed to  utilize  the  above  reaction  for  a  toxicological  test. 
In  1836  Marsh4  published  his  celebrated  memoir,  entitled  "Ac- 
count of  a  Method  of  Separating  Small  Quantities  of  Arsenic  from 
Substances  with  which  it  may  be  Mixed."  He  elaborated  and  sim- 
plified the  apparatus,  generated  hydrogen  by  means  01  dilute  sul- 
phuric acid  and  zinc,  inflamed  the  escaping  gas,  and  deposited  the 
arsenic  in  the  form  of  metal,  which  afterward  could  be  converted 
into  arsenous  oxide.  This  brief  review  amply  shows  that  Marsh 
was  not  the  discoverer  ot  the  arsenic  test,  which  is  usually  called  by 
his  name,  but  like  many  other  useful  processes  and  inventions,  it 
seems  to  have  been  gradually  evolved  by  the  combined  efforts  of 
many  minds.  This,  however,  must  be  said  :  Marsh  was  the  first 
to  simplify  and  prominently  bring  forward  the  test  very  much 
improved,  if  not  perfected. 
Little,  probably,  did  these  early  investigators  think  that  this 
method  would  be  more  thoroughly  investigated  than  any  other  proc- 
ess in  the  realms  of  chemistry. 
Many  other  methods  have  been  suggested  from  time  to  time,  but 
not  one  of  them  as  yet  has  been  proved  the  superior  or  even  the 
equal  of  Marsh's  test  modified  in  one  form  or  another. 
1 1775,  Memoires  de  Scheele,  t.  I,  170  :  Om  Arsenick  och  dess  Syra;  Kongl. 
Svenik.  Vatenikops  Academiens  Handlingar  :  Ar.  1775,  V.  36,  265. 
2  1798,  Ann.  de  Chem.y  28,  213;  1800,  Jour,  de  Phys.  et  Chem.,  51,  173. 
3  1803,  Nicholson's  Journal,  6,  200;  from  Royal  Academy  of  Sciences,  Berlin, 
P-  370. 
4 1836,  The  Edinburgh  New  Philo.  Journal,  21,  229;  1^37,  Jour,  de  Pharm., 
23,  553;  Ann.  (Liebig),  23,  p.  207, 
