Am.  Jour.  Pharm.  \ 
August,  19 18.  » 
Colloidal  Metals. 
595 
paratus  or  accurate  quantitative  solutions  of  any  kind.  As  noted 
by  Clark  and  Lubs,  the  use  of  pairs  of  test  tubes  is  a  device  which 
is  "  optically  very  imperfect,  'but  it  works  very  well." 
Titration  of  Mediums. — One  Cc.  of  medium  to  be  titrated  is 
added  to  4  Cc.  of  distilled  water  in  a  test  tube.  Ten  drops  of  indi- 
cator are  added,  the  color  is  compared  with  the  color  standards  if 
it  is  desired  to  determine  the  initial  reaction,  and  titration  to  the 
desired  hydrogen  ion  concentration  with  twentieth-normal  sodium 
hydroxide  is  performed.  Fifty  times  the  amount  used  will  repre- 
sent the  amount  of  normal  sodium  hydroxide  to  be  added  to  1  liter 
of  medium.  If  in  carrying  out  the  titration  sufficient  sodium  hy- 
droxide solution  is  added  so  that  the  indicator  color  is  appreciably 
diluted,  the  end-point  tubes  of  the  comparator  should  be  filled  to  a 
similar  volume  before  the  final  comparison  is  made.  If  desired, 
in  comparing  colors,  use  may  be  made  of  the  block  described  by 
Hurwitz,  Meyer  and  Ostenberg,5  though  practically  it  has  not  been 
found  to  increase  the  accuracy  appreciably.  This  is  a  wooden  block 
in  which  two  pairs  of  adjacent  holes  have  been  drilled  to  receive 
two  pairs  of  test  tubes.  The  holes  are  connected  hy  slits  so  that 
each  pair  may  be  viewed  by  transmitted  light.  In  this  procedure 
one  pair  of  holes  would  contain  the  acid-alkali  pair  chosen  for  an 
end-point,  and  the  other  the  diluted  mediums  and  a  tube  of  distilled 
water.  Compensation  for  colored  fluids  can  usually  be  accomplished 
according  to  the  principle  introduced  by  Walpole,6  by  using  in  one 
row  of  the  color  standard  series  5  Cc.  amounts  of  the  fluid  made 
acid  or  alkaline. 
COLLOIDAL  METALS.1 
By  Thomas  Stephenson,  F.R.S.,  Edinburgh. 
During  the  last  decade  considerable  attention  has  been  devoted 
to  the  peculiar  catalytic  action  of  the  metals  in  colloidal  solution,  and 
the  application  of  this  action  to  therapeutics.  The  nature  and  prop- 
erties of  colloidal  solutions  have  long  been  known,  but  the  assump- 
tion by  metallic  substances  of  the  colloidal  state  is  a  comparatively 
recent  discovery.    It  is  a  good  many  years  since  colloid  silver  was 
5  Hurwitz,  Meyer  and  Ostenberg,  Bull.  Johns  Hopkins  Hosp.,  1916,  27,  16. 
6  Walpole,  Biochem.  Jour.,  1910-1911,  5,  207. 
1  From  the  Prescriber,  June,  191 8, 
