LIQUID  DIFFUSION  APPLIED  TO  ANALYSIS. 
515 
probable  primary  source  of  the  force  appearing  in  the  phenomena 
of  vitality,  as  living  matter  without  form.  To  the  gradual 
matter  also  in  which  colloidal  changes  take  place  (for  they 
always  demand  time  as  an  element),  may  the  chronic  nature  and 
periodicity  of  vital  phenomena  be  ultimately  referred. 
For  the  separation  of  unequally  diffusive  crystalloids  from 
each  other,  jar  diffusion  was  had  recourse  to.  The  mixed  so. 
lution  was  conveyed  by  means  of  a  pipette  to  the  bottom  of  a 
column  of  water  contained  in  a  cylindrical  glass  jar.  A  kind 
of  cohobation  takes  place,  a  portion  of  the  most  diffusing  sub- 
stance rising  and  separating  from  the  less  diffusive  substances, 
more  and  more  completing  as  it  ascends. 
The  separation  of  a  crystalloid  from  a  colloid  is  more  properly 
effected  by  a  combination  of  diffusion  with  the  action  of  a  sep- 
tum composed  of  an  insoluble  colloidal  material.  Animal 
membrane  will  serve  for  the  latter  purpose,  or  a  film  of  gelatin- 
ous starch,  hydrated  gelatin  itself,  albumen,  or  animal  mucus. 
But  by  much  the  most  effective  septum  used  was  paper,  as  it  is 
metamorphosed  by  sulphuric  acid  (Gaine).  It  is  now  supplied 
by  Messrs.  De  la  Rue,  and  has  become  familiar  under  the  name 
of  "  vegetable  parchment  "  or  parchment  paper."  From  sheet 
gutta  percha  a  flat  hoop  is  formed,  eight  or  ten  inches  in  di- 
ameter by  three  inches  in  depth,  and  one  side  is  covered  by  a 
disc  of  parchment-paper,  so  as  to  form  a  vessel  like  a  sieve.  A 
mixed  solution,  which  may  be  supposed  to  contain  sugar  and 
gum,  is  placed  upon  the  septum  to  a  depth  of  half  an  inch,  and 
the  instrument  then  floated  upon  a  considerable  volume  of  water 
contained  in  a  basin.  Three.fourths  of  the  sugar  diffuses  out  in 
twenty-four  hours,  and  so  free  from  gum  as  to  be  scarcely  affected 
by  subacetate  of  lead,  and  to  crystallize  on  the  evaporation  of 
the  external  water  by  the  heat  of  a  water.bath. 
The  unequal  action  of  the  septum,  which  causes  the  separa- 
tion described,  appears  to  depend  on  this: — The  crystalloid 
sugar  is  capable  of  taking  water  from  the  hydrated  colloidal 
septum,  and  thus  obtains  a  medium  for  diffusion ;  but  the 
colloid  gum  has  little  or  no  power  to  separate  the  combined 
water  of  the  same  septum^  and  does  not  therefore  open  the 
door  for  its  escape  by  diffusion,  as  the  sugar  does.  This  sepa- 
rating action  of  the  colloidal  septum  is  spoken  of  as  dialysis. 
