THE  ENGLISH  COMMERCIAL  SODA  TEST. 
447 
The  author  is  at  present  engaged  to  ascertain  what  substances 
are  capable  of  retarding  or  preventing  the  development  of  these 
germs.  As  to  the  conclusions  derived  from  the  above  observa. 
tions,  Dr.  Heisch  thinks  that  wherever  the  described  germs 
occur  in  water  they  are  distinct  evidence  of  sewage-contamina- 
tion.— Proc.  Chem.  jSoc,  in  Chem.  News^  June  24,  1870. 
DOUBLE  SULPHIDE  OF  POTASSIUM  AND  IRON. 
By  Professor  Morton. 
By  heating  an  intimate  mixture  of  5  parts  of  sulphur,  5  of 
potassic  carbonate  and  1  part  of  fine  iron  filings,  C.  Preiss  has 
succeeded  in  forming  a  double  sulphide  of  potassium  and  iron, 
which  crystallizes  in  red  needles,  has  a  metallic  lustre,  and  re- 
sembles in  appearance  potassic  permanganate.  Its  formula  is 
KS,  FE^Sg.*  The  same  compound  has  been  obtained  independ_ 
ently  by  R.  Schneider,  who  also  has  formed,  by  the  replacement 
of  iron  by  bismuth,  the  analogous  compound,  KSjBigSg. — Pogg. 
Ann.,  from  Chem.  News,  Lond.,  June  17,  1870. 
THE  ENGLISH  COMMERCIAL  SODA  TEST. 
To  the  Editor  of  the  Chemical  News  : 
Sir, — My  attention  has  lately  been  drawn  to  a  strange  error 
made  by  some  analysts  in  attempting  to  apply  the  English  com- 
mercial test  for  soda  to  samples  of  alkali,  soda-ash,  &c.,  the 
result  of  which  error  is  to  make  the  test  indicate  from  1  to  \^ 
per  cent,  more  soda  than  the  sample  contains  by  the  proper  Eng- 
lish test.  It  is  well  known  that  this  (the  English  soda-test)  had 
its  origin  in  the  early  days  of  the  soda  trade — when  chemists 
believed  the  equivalent  of  soda  to  be  32,  and  that  of  carbonate 
of  soda  54 ;  and  that,  consequently,  test  acid  was  made  so  that 
40  parts  of  sulphuric  acid  neutralized  54  parts  of  carbonate  of 
soda,  equal  to  32  of  soda.  This  method  of  testing  has  always 
been  and  still  is  used  by  the  soda  trade  throughout  England,  and 
it  is  a  custom  well  understood  by  both  buyers  and  sellers.  It 
indicates  0-66  per  cent,  more  soda  in  a  50  per  cent,  alkali  than 
the  rigidly  correct  test  based  on  the  new  equivalent  31  would 
*  Journ.  f.  Prukt.  Chemie,  evil,  p.  10. 
