Scheikunde.  — De  Heer  Ernst  Cohen  biedt  eene  mededeeling  aan 
van  den  Heer  Nii,  Ratan  Dhar:  "Catalysl'^  — Part  VI  — 
Temperature  coefficimts  of  keterogeneoii.s  reactims.” 
(Mede  aangeboden  door  den  Heer  van  Romburgh). 
In  foregoing  papers*),  tbe  temperature  coetïicients  of'  catalysed 
and  uncatalysed  reactions  in  a lioinogeneons  medium  have  been 
studied.  It  lias  been  shown  that  a positive  ealalyst  produces  a lower- 
ing  in  the  value  of  the  temperature  coëfficiënt  of  the  reaction,  the 
reverse  is  llje  case  with  a negative  catalyst.  It  w'as  also  proved  that 
the  higher  the  ordei'  of  a reaction,  the  smaller  is  the  temperature 
coëfficiënt. 
The  object  of  this  paper  is  to  discuss  the  experimental  researches 
and  tind,  if  |)ossible,  similar  relations  in  the  domain  of  heterogen- 
eous  reactions. 
In  order  to  make  clear  the  question  of  the  tem|»erature  coefficients 
of  lieterogeneous  reactions,  it  is  necessary  to  iiidicale  briefly  their 
characteristics. 
In  a i-eaction  between  a liquid  and  a solid,  according  to  the 
diffusiou  theory  of  reaction  velocity  a thin  layer  of  liquid  adhering 
to  the  solid  remains  unafifected  by  stirring  and  the  reaction  is  main- 
tained  by  the  ti'ansport  of  dissolved  substances  across  this  layer  of 
diffusiou.  Moi'eover,  it  is  assumed  that  at  the  boundary  surface 
between  tvvo  phases.  the  velocity  of  the  Chemical  reaction  is  extreme- 
ly  high.  When  the  difï'usion  is  sufficiently  slow  compared  with  the 
olher  stages  of  the  reaction  the  velocity  of  the  whole  reaction  will 
be  determined  by  the  rate  of  diffusion  alone. 
This  theoiy  was  tirst  proposed  by  Noyes  and  Whitney^)  for  some  j 
special  cases,  bul  its  general  applicability  to  \ arious  types  of  hetero-  j 
geneous  reactions  was  indicated  by  Neknst  and  Brunner  ®),  and  lias 
since  been  accepted  by  several  investigalors  as  giving  the  best  ex-  ! 
planation  of  facts  in  heterogeneous  systerns.  j 
On  the  other  hand,  the  general  applicability  of  the  diffusiou  theory  > 
H Jour.  chera.  soc-  1917,  lil,  7U7;  Annales  de  chimie,  1919.  | 
Zeit.  Phys.  Chem.  1897,  23,  689. 
b ibid.  J904  47,  52,  56.  | 
