314 
REMARKS  ON  DIALYSIS. 
sodium  will,  under  the  circumstances,  diffuse  out  of  the  lower 
stratum  into  the  upper,  just  as  quickly  as  though  the  two  strata 
were  water  and  saline  water. 
Now  if  the  lower  stratum  of  jelly  be  mixed  with  ten  per  cent 
of  vegetable  extractive  instead  of  the  chloride  of  sodium,  no 
diffusion  will  occur,  that  body  from  its  colloidal  nature  not 
being  able  to  traverse  the  gelatinous  medium  as  did  the  salt. 
Taking  advantage  of  this  fact,  Mr.  Graham  found  on  trial 
that  a  very  thin  stratum  of  a  gelatinous  medium  would  prevent 
the  diffusion  of  colloid  bodies,  and  that  by  the  use  of  such  a 
septum  or  diaphragm,  the  process  of  separating  the  crystalloids 
from  the  colloids  became  greatly  simplified.  After  a  trial  of  va- 
rious gelatinous  tissues,  parchment  paper*  was  found  in  all  re. 
spects  the  best,  although  paper  sized  with  starch  jelly,  or  gela- 
tin, or  coagulated  albumen,  would  answer. 
When  the  process  of  diffusion  is  thus  used  as  a  means  of  sepa- 
rating bodies  by  the  aid  of  a  hydrated  septum,  the  process  is 
called  by  Mr.  Graham  dialysis,  the  instrument  a  dialyser  and 
the  solution  of  the  matter  which  passes  the  septum  the  diffusatey 
just  as  we  use  the  terms  filtrate  and  percolate  as  substantives. 
The  form  of  dialyser  recommended 
by  Mr.  Graham,  consists  of  a  short 
cylinder  six  to  ten  inches  in  diame- 
ter, and  two  inches  high,  made  of  a 
band  of  gutta  percha.  Over  one  end 
of  this  a  sheet  of  parchment  paper  is 
stretched,  and  tightly  tied  by  a  string, 
the  edges  of  the  paper  being  crimped 
around  the  outside  of  the  cylinder  so  as  to  extend  some  distance 
above  the  string  as  at  a  in  the  figure.  This  part  of  the  appa- 
ratus which  is  to  contain  the  substance  to  be  diffused,  is  placed 
within  a  larger  glass  vessel,  of  similar  shape,  about  six  inches 
deep,  and  capable  of  holding  five  to  ten  times  as  much,  as  at  b. 
Into  this,  distilled  water  is  poured  until  three-fourths  full,  the 
smaller  vessel  floated  on  it,  and  the  liquid  to  be  diffused  then 
*  Parchment  paper  is  made  by  simply  passing  ordinary  white  wrapping 
paper  quickly  through  a  cool  mixture  of  two  parts  of  sulphuric  acid,  and 
one  of  water  by  weight,  and  then  washing  it  thoroughly  till  the  acid  ia 
removed. 
/ 
