102 
Dialysis. 
Am.  Jour.  Pharm. 
Mar.,  1878. 
by  ferric  chloride.  As  the  proportion  of  these  two  ferric  compounds 
can  be  changed  at  will,  a  chemical  combination  of  them  cannot  be  well 
thought  of. 
Louisville,  February,  1878. 
DIALYSIS. 
By  Richard  V.  Mattison,  Ph.G. 
(Read  at  the  Alumni  Meeting,  February  7.) 
It  would  be  a  matter  of  curious  interest  to  know  how  many  times 
in  the  past  few  months  the  question,  What  is  dialyzed  iron  ?  what  do 
you  mean  by  dialyzed?  has  been  asked  and  answered;  answered  in 
many  instances,  we  fear,  in  a  very  unsatisfactory  manner,  since  the 
explanation  of  the  true  meaning  of  dialysis  escapes  many  because  of  its 
very  simplicity.  It  means  simply  separation ;  a  separation  which  differs 
from  filtration,  however,  in  this  respect :  that,  while  the  latter  is  a 
separation  of  soluble  from  insoluble  substances,  the  former  is  a  separa- 
tion of  substances  soluble  in  the  same  media  in  common,  but  differing 
from  each  other  in  their  different  difFusibility.  By  diffusibility  we  mean 
the  power  possessed  by  certain  bodies  of  passing  through  animal  or 
vegetable  membranes.  With  gases  this  diffusive  power  has  long  been 
well  known,  and  Graham  long  ago  laid  down  the  rule  that  "  the  diffu- 
sive power  of  all  gases,  simple  and  compound,  varies  inversely  with  the 
square  root  of  the  density  of  the  gas  itself." 
The  diffusive  power  of  solids  follows  no  definite  rule.  Solids  of  the 
same  chemical  constitution  diffuse  with  the  most  unequal  velocity  ;  for 
instance,  if  we  make  a  dense  solution  of  a  mixture  of  sodium  and  potas- 
sium chlorides,  and  pour  carefully  on  the  solution  a  stratum  of  distilled 
water,  in  a  few  hours  the  latter  will  contain  a  large  proportion  of  the 
potassium  chloride,  but  scarcely  a  trace  of  the  sodium  salt.  The 
ammonium  chloride  diffuses  still  more  rapidly,  hence  it  is  readily  seen 
that  the  rate  of  diffusion,  or,  if  you  will,  dialysis,  varies  without  appar- 
ent cause.  To  a  certain  limit,  which  is  found  in  practice  to  average 
about  5  per  cent.,  differing,  however,  with  different  salts,  the  rate  of 
diffusion  increases  with  the  density  of  the  solution,  and  hence  the  fact 
that  chemical  solutions,  e.  g.,  the  mixture  of  ferric  chlorides  and  oxy- 
chlorides,  dialyze  rapidly  at  first,  but  afterward  more  slowly. 
We  are  all  familiar  with  the  diffusion  occurring  in  the  porous  cell  of 
