616  Diffusion  of  Aeids  and  Bases.        { A%Je0c^imarm" 
When  the  hydrochloric  acid  solution  contains  only  ts  gram-mol., 
and  the  ammonia  1  rnol.,  the  surface  of  separation  falls  3*9,  "8*2,  and 
11*9  mm.  in  one,  four,  and  nine  hours  respectively  ;  with  solutions 
containing  1*5  gram-mol.  of  hydrochloric  acid  and  12  gram-mols.  of 
ammonia  respectively,  the  surface  of  separation  falls  3'9,  8*4,  and 
11*6  mm.  in  the  same  times.  In  those  experiments  in  which  the  * 
region  occupied  by  the  ammoniacal  liquid  extends  downwards,  the 
two  solutions  do  not  remain  so  sharply  separated,  and  the  surface  of  ' 
separation  is  often  irregular. 
Jn  one  experiment  with  solutions  containing  1  gram-mol.  of  soda 
and  tb-  gram-mol.  of  acetic  acid  respectively,  the  region  occupied  by 
the  alkaline  liquid  extended  5*8,  11*6,  and  17*4  mm.  upwards  in  one, 
four,  and  nine  hours,  and  28*5,  40*5,  and  39*7  mm.  in  one,  two,  and 
three  days  respectively. 
The  apparatus  employed  in  these  experiments  consists  of  two  grad- 
uated glass  tubes  (120  mm.  in  length,  7  mm.  in  internal  and  23  mm. 
external  diameter),  fixed,  with  their  open  ends  turned  towards  each 
other,  on  a  perpendicular  brass  stand,  so  that  they  can  be  moved  up 
and  down  or  rotated  round  the  stand.  The  open  ends  have  ground 
edges,  the  other  extremities  being  closed  with  glass  plates,  fastened  on 
with  cement.  The  underneath  tube  is  filled  in  situ ;  the  other  is 
filled,  then  inverted,  and  any  liquid  which  falls  out  replaced  with  a 
small  pipette.  At  the  commencement  of  the  experiment  the  open 
ends  of  the  two  tubes  are  brought  to  the  same  level,  and  they  are  then 
rotated  until  the  one  is  exactly  over  the  other. 
The  author  discusses  the  phenomena  of  diffusion  mathematically, 
and,  from  the  data  given  above,  the  coefficient  of  diffusion  of  am- 
monia is  found  to  be  0*92,  and  that  of  hydrochloric  acid  3*02. 
From  observations  made  in  a  similar  manner  with  ammonia  and 
various  acids,  the  diffusion  coefficients  are  found  to  be  as  follows  : —  * 
Nitric  acid,  2'93  ;  sulphuric  acid,  1*82;  oxalic  acid,  1*14;  and  acetic 
acid,  0  88.  In  all  the  experiments,  solutions  containing  one  equiva- 
lent of  the  acid  and  rV  of  an  equivalent  of  ammonia  respectively  were 
employed.  In  two  experiments  in  which  solutions  of  one  equivalent 
of  base  and  ts  of  an  equivalent  of  acetic  acid  were  employed,  the 
diffusion  coefficient  of  potash  was  found  to  be  0'73,  and  that  of  soda 
1*57.    The  above  values  are  for  the  temperature  of  21°. 
The  diffusion  coefficients  of  hydrochloric  acid,  nitric  acid,  and  sul- 
phuric acid  are  considerably  larger  than  those  found  by  Scheffer  (Zeit. 
