516 
LIQUID  DIFFUSION  APPLIED  TO  ANALYSIS. 
Dialysis  was  applied  to  the  preparation  of  various  colloids. 
The  mixed  solution  obtained  by  pouring  silicate  of  soda  into 
water  acidulated  with  hydrochloric  acid,  was  placed  upon  a 
parchment-paper  dialyser  and  allowed  to  diffuse  into  water,  the 
latter  being  occasionally  changed.  After  the  lapse  of  five  days 
seven-eighths  of  the  original  silicic  acid  was  found  to  remain 
liquid  upon  the  septum,  and  to  be  so  free  from  hydrochloric  acid 
and  chloride  of  sodium  as  not  to  give  a  precipitate  with  acid 
nitrate  of  silver.  The  true  hydrated  alumina,  and  also  Mr. 
Crum's  metalumina,  were  obtained  soluble  by  dialysing  solutions 
of  these  oxides  in  the  chloride  and  acetate  of  the  same  metal. 
So  also  the  hydrated  peroxide  of  iron,  in  addition  to  the  hydrated 
metaperoxide  of  iron  of  M.  Pdan  de  Saint  Gilles,  and  the  solu- 
ble hydrated  chromic  oxide.  The  varieties  of  prussian  blue  are 
obtained  soluble  by  dialysing  their  solution  in  oxalate  of  am- 
monia, the  latter  salt  diffusing  away.  Stannic  and  titanic  acids 
appear  as  insoluble  gelatinous  hydrates. 
A  solution  of  gum-arabic  (gummate  of  lime),  dialysed  after 
an  addition  of  hydrochloric  acid,  gave  at  once  the  pure  gummic 
acid  of  Fremy.  Soluble  albumen  is  obtained  in  a  state  of  purity 
by  dialysing  that  substance  with  an  addition  of  acetic  acid. 
Caramel  of  sugar,  purified  by  repeated  precipitation  by 
alcohol  and  afterwards  by  dialysis,  contains  more  carbon  than 
any  of  the  caramelic  bodies  of  G^lis ;  it  forms  a  tremulous 
jelly  when  concentrated,  and  appears  decidedly  colloidal.  Cara- 
mel, like  all  other  colloids,  has  a  soluble  and  an  insoluble 
modification.  The  latter  has  its  solubility  restored  by  the  action 
of  alkali,  followed  by  that  of  acetic  acid  and  subsequent  dialy- 
sis. 
Dialysis  proves  highly  useful  in  separating  arsenious  acid 
and  metallic  poisons  from  organic  fluids.  Defibrinated  blood, 
milk,  and  other  organic  fluids  charged  with  a  few  milligrammes 
of  arsenious  acid,  and  placed  upon  the  dialyser,  were  found  to 
impart  the  greater  proportion  of  the  arsenious  acid  to  the  ex- 
ternal water  in  the  course  of  twenty-four  hours.  The  diffusate 
was  so  free  from  organic  matter,  that  the  metal  could  be  readily 
precipitated  by  sulphuretted  hydrogen,  and  the  quantity  weighed. 
Ice  at  or  near  its  melting-point  appears  to  be  a  colloidal 
substance,  and  exhibits  a  resemblance  to  a  firm  jelly  in  elasticity, 
the  tendency  to  rend,  and  to  redintegrate  on  contact. 
