626 
Colloidal  Metals. 
Am.  Jour.  Pharm. 
Sept.  19 19. 
charged  with  positive  or  negative  electricity,  and  the  passage  of  an 
electric  current  through  the  solution  causes  the  particles  to  move 
respectively  towards  the  cathode  and  the  anode.  Some  colloidal 
solutions  are  readily  precipitated  by  heat  or  by  mechanical  means, 
and,  as  will  be  seen  later,  this  has  to  be  guarded  against  in  their 
therapeutic  application. 
The  nature  and  properties  of  colloids  and  of  colloidal  solutions 
have  been  known  for  some  time,  but  the  assumption  by  the  metals 
and  metalloids  of  the  colloidal  state  is  a  comparatively  recent  dis- 
covery. It  is  these  that  have  come  into  extensive  use  in  medicine. 
The  first  metal  to  be  used  in  this  way  was  colloidal  silver,  which,  in 
1896,  was  introduced  into  therapeutics  under  the  name  of  "coll- 
argol." This  substance,  which  occurs  in  small  black  scales  having  a 
metallic  lustre,  forms  with  water  an  opaque  solution,  which  has  all 
the  properties  of  a  colloid.  Collargol,  however,  is  not  really  a 
colloidal  metal,  but  is  generally  considered  to  be  a  combination  of 
an  acid  silver  molecule  with  ammonia,  i.  e.,  collargolate  of  ammonia. 
Shortly  after  the  introduction  of  collargol,  Trillet  succeeded  in 
preparing  oxydases  of  certain  metals  by  precipitating  solutions  of 
metallic  salts  with  an  alkali  in  presence  of  albumin,  forming  a  kind 
of  colloidal  solution  of  the  metals.  Later,  Bredig  produced  the 
solutions  known  by  his  name.  These  are  true  colloidal  "  solutions  " 
(or  suspensions)  of  the  metals,  and  are  produced  by  passing  an 
electric  current  through  pure  water  between  electrodes  of  the  metal 
to  be  dissolved.  The  current  diffuses  a  minute  quantity  of  the  metal 
throughout  the  liquid — the  metal,  in  fact,  becomes  volatilized  in  the 
liquid.  The  resulting  solution  is  in  every  case  a  dichroic  liquid, 
transparent  to  transmitted  light,  and  opaque  to  reflected  light.  Sus- 
pended particles  can  be  detected  only  by  the  ultra-microscope,  and 
the  solution  in  all  respects  obeys  the  rules  laid  down  for  colloidal 
substances.  The  metal  is  in  a  state  of  very  minute  subdivision,  and 
the  particles  possess  that  vibratory  motion  known  as  "  Brownian 
movement."  Different  metals  have  been  used  in  the  preparation  of 
such  solutions,  and  more  recently  colloidal  solutions  of  the  metal- 
loids, such  as  sulphur  and  iodine,  have  also  been  prepared. 
In  addition  to  the  electrical  method  for  the  preparation  of 
colloids,  there  is  the  chemical  method  which  has  come  into  fairly 
general  use  also.  This  usually  consists  in  the  reduction  of  a  metallic 
salt  by  a  suitable  agent  in  the  presence  of  a  protective  colloid  such 
