1 145 
resull  tliat  the  liomogeaeoiis  eheuiioal  i-eaetioii  iii  the  adsorbed  layer 
is  the  real  process  of  whicli  we  detennine  the  temperatnre  coëfficiënt. 
Now  it  is  interesting  (o  observe  Ihat  Bredig  and  Teletofe  (Zeit. 
Elektrochem.  1906,  12,  583)  have  calcnlated  the  thickness  of  ihe 
diffusion  lajer  from  the  Nernst  diffusion  expression  (Zeit.  Plijs.  Cliein. 
1888,  2,  634),  utilising  the  data  obtained  from  the  deeomposition 
of  hydrogen  peroxide  in  pi-esence  of  colloidal  platinnm  and  fonnd 
the  tliickness  to  be  0.05  mm.  i.e.  of  the  same  ordei'  as  Hrunner 
fonnd  in  the  case  of  the  dissolntion  of  benzoic  acid  in  water.  This 
seems  to  show  tliat  in  spite  of  the  Brownian  movement  the  ditfnsion 
lajer  remains  unchanged.  If  this  is  trne,  the  abo\e  explanalion  of 
the  high  temperature  coëfficiënt  in  the  case  of  colloids  and  enzymes 
breaks  down.  Moreover,  on  this  |)oint  of  view,  the  high  valnes  of 
the  temperature  coëfficiënt  obtained  in  the  oxidations  of  oxalic  acid 
and  phenylthiocarbamide  in  presence  of  the  solid  catalyst  blood- 
charcoal,  i-emain  entirely  unexplained. 
Looking  at  the  whole  problem,  it  seems  probable  tliat  in  some 
cases  the  slow  Chemical  change  aftects  the  velocity  of  tlie  total 
reaction,  wliilst  in  other  cases,  ditfnsion  plays  the  most  important 
role  and  it  is  desirable  to  investigate  fully  the  kinetics  of  each 
individnal  case. 
(fertainly  mnch  light  wonld  be  thrown  on  the  whole  qnestion  if 
we  can  stndy  the  kinetics  and  temperature  coefücients  of  one  and 
the  same  reaction  without  any  catalyst  and  in  presence  of  both 
homogeneous  and  heterogeneons  catalysts.  The  velocity  of  decom- 
position  of  hydrogen  peroxide,  tbr  example,  may  be  investigated  at 
varions  temperatnres  (1)  without  any  catalyst  and  (2)  in  presence 
of  iodides  or  any  other  substance  solnble  in  water  (compare  Bredig 
and  Walton.  Zeit.  Phys.  Ohem.  1904,  47.  185)  and  (3)  in  presence 
of  colloidal  metals,  MnO,,  charcoal,  solid  metals  etc. 
Ldhoraioire  de  Chimie  Allnérale,  Sorhonne,  Faris. 
Imperia I College  of  Science.  London  S.  IV.  7. 
