76 
Colloids  and  Crystals. 
I  Am.  Jour.  Pharm. 
\   February,  1914. 
suspension,  which  settles  at  once.  Gold  is  an  irreversible  colloid. 
The  distinction  is  fundamental.  Many  organic  colloids  are  reversible, 
while  it  is  rather  the  habit  of  the  inorganic  colloids  to  behave  in  the 
irreversible  way. 
IV. 
In  order  to  prepare  a  sol  containing  an  irreversible  colloid  all 
that  is  necessary  is  to  reduce  the  solid  to  extreme  subdivision  in  a 
liquid  in  which  it  is  insoluble.  The  electric  arc  furnishes  a  rapid 
and  simple  method.5  Two  gold  wires  about  2  mm.  thick  are  con- 
nected with  a  220-volt  circuit  and  brought  together  under  distilled 
water.  A  no-volt  circuit  can  be  used,  but  more  patience  is  required. 
Sols  of  platinum,  silver,  copper,  and  other  metals  can  be  made  in  the 
same  way.  By  related  electrical  methods,  using  such  liquids  as  pen- 
tane  and  anhydrous  ether,  Svedberg  6  obtained  sols  of  all  five  of  the 
alkali  metals.  The  colors  of  the  sols  agreed  with  those  of  the  vapors 
of  the  corresponding  metals. 
Chemical  reduction  of  a  salt  of  a  metal  furnishes  another  method 
which  has  been  largely  employed  by  Zsigmondy  7  and  other  investi- 
gators. For  instance,  a  very  dilute  solution  of  auric  chloride  is  mixed 
with  such  reducing  agents  as  formaldehyde,  hydroxylamine  or  an 
ethereal  solution  of  phosphorus.  The  gold  sols  obtained  in  this  way 
are  usually  red  by  transmitted  light,  the  particles  being  bright  green 
and  very  much  smaller  than  in  the  sols  obtained  by  the  electrical 
method. 
By  various  chemical  methods,  which  lack  of  space  forbids  us  .to 
discuss,  sols  of  sulphides  (CdS,  As2S;!,  Sb2S3,  etc.)  and  oxides 
(Fe203,  A120;!)  can  be  obtained.  The  sol  of  aluminum  oxide  is  im- 
portant on  account  of  its  connection  with  dyeing  and  mordanting. 
The  formation  of  the  blood-red  sol  of  ferric  oxide  by  adding  a  con- 
centrated solution  of  ferric  chloride  to  about  50  volumes  of  boiling 
distilled  water  is  a  simple  and  beautiful  lecture  experiment. 
In  making  colloidal  solutions  of  salts,  the  essential  thing  is  to 
mix  dilute  solutions  of  the  precipitants,  using  a  liquid  in  which  the 
5  Bredig,  Zcitschrift  fur  angeivandte  Chemie,  1898,  p.  951.  For  a  full 
account  of  Bredig's  work  with  the  platinum  sol  see  Zcitschrift  fur  physikal- 
ischc  Chcmic,  vol.  31,  pp.  258-353  (1899). 
6  Bcrichtc  dcr  dcutschcn  chcmischen  Gcsellschaft,  vol.  38,  p.  3616  (1905). 
7  See  his  monograph,  "  Zur  Erkentniss  der  Kolloide  "  (Jena,  1905),  which 
has  been  translated  by  Jerome  Alexander. 
