316 
REMARKS  ON  DIALYSIS. 
used  in  air  pump  experiments,  in  lieu  of  the  gutta  percha 
cylinder,  attaching  the  parchment  paper  to  the  larger  end. 
This  he  supports  on  a  large-sized  beaker  glass,  into  which  it 
enters  just  sufficiently  to  be  half  immersed  in  the  water  it  con- 
tains. The  liquid  to  be  diffused  is  then  poured  in  the  small 
upper  end,  which  may  then  be  covered  with  a  glass  plate  to 
keep  out  dust  and  prevent  evaporation. 
When  the  object  is  to  get  the  diffusate  as  concentrated  as 
possible,  it  is  necessary  to  use  a  shallow  vessel,  as  in  the  figure, 
and  renew  the  water  when  the  process  is  checked;  but  when  the 
object  is  rather  to  exhaust  the  liquid  above  the  septum  of  its 
diffusible  matter  without  reference  to  the  state  of  dilution  of  the 
diffusate,  then  a  deep  vessel  is  best.  Prof.  Redwood  has  in- 
vented a  grooved  plate  by  which  a  thin  stream  of  water  is  made 
to  take  a  spiral  course  around  the  outer  surface  of  the  parch- 
ment paper,  from  the  circumference  to  the  centre,  so  as  to  con- 
stantly renew  the  liquid  in  contact  with  the  septum.  There 
may  be  an  advantage  in  certain  cases  arising  out  of  this  arrange- 
ment, yet  the  process  of  dialysis  is  necessarily  so  gradual  that 
the  simple  arrangement  of  the  figure  will  probably  succeed 
in  most  instances. 
It  now  remains  to  remark  on  the  practical  applications  and 
uses  of  dialysis,  as  they  have  been  thus  far  developed. 
1st.  As  an  agency  in  proximate  analysis;  affording  a  means 
of  separating  crystallizable  and  well  defined  bodies  ff  om  amor- 
phous matters,  like  extractive,  gums,  mucilages,  the  pectin  like 
substances,  the  tannins,  oils,  etc.  For  instance,  if  a  vegetable 
extract  containing  an  alkaloid  or  crystalline  neutral  bitter 
principle,  be  rubbed  down  with  water  to  a  uniform  mixture  and 
thrown  on  the  dialyser,  this  active  principle,  together  with  all 
the  soluble  earthy  matter  will  be  found  in  the  diffusate,  whilst  the 
inert  ingredients  will  be  retained.  Mr.  John  Attfield  (Pharm. 
Jour.  March,  1862,  p.  4 17)  has  applied  this  apparatus  in  an  inves- 
tigation of  the  mineral  constituents  of  plants,  especially  as  ex- 
hibited in  the  saline  efflorescences  on  the  vegetable  extracts,  so 
familiar  to  all  apothecaries.  Mr.  Attfield  also  suggests  its  use 
for  purifying  crude  lemon  juice. 
Mr.  Graham  has  separated  the  crystallized  ingredients  of 
urine.  Baron  Liebig  has  isolated  creatin  from  extract  of  meat ; 
