Am.  Jour.  Pharm.  \ 
Aug.,  1886.  J 
Contact  Actions. 
381 
Characteristic  Reaction  for  Citric  Acid. — To  distinguish  citric  acid 
from  malic  acid  or  tartaric  acid,  M.  Mean  fuses  together  1  gram  of 
citric  acid  and  70  centigrams  of  glycerin  in  a  porcelain  capsule  by 
cautiously  heating  until  the  mass  begins  to  bubble  up  and  emit 
vapors  of  acrolein;  on  adding  a  small  quantity  of  ammonia  and 
about  2  drops  of  fuming  nitric  acid,  or  a  10-per-cent.  solution  of 
hydrogen  peroxide  a  bright  green  coloration  is  produced,  which 
gradually  turns  blue  if  the  heat  is  continued.  Neither  malic  acid 
nor  tartaric  acid  show  the  same  reaction. — Journal  de  Pharm.  et  de 
Chimie,  1886,  xiii,  p.  477. 
CONTACT  ACTIONS. 
By  D.  Mendelejeff. 
It  is  a  generally  received  opinion  that  not  only  the  molecules  as  a 
whole,  but  each  of  the  constituent  atoms  are  in  motion,  the  movement 
of  the  latter  being  conditioned  by  the  limits  of  the  molecule.  If  the 
conception  be  reasonable,  then  in  every  state  of  aggregation  there 
are  presumably  motions  of  two  kinds,  the  one  internal,  the  other 
superficial.  The  contact  of  two  molecules,  as  also  conditions  of 
temperature,  pressure,  and  electric  potential,  influence  each  kind 
of  motion;  thus  the  phenomenon  of  catalytic  or  contact  action 
gradually  passes  into  that  of  dissociation  or  of  decomposition. 
Herein  the  decomposition  of  hydrogen  peroxide  is  analogous  to 
that  of  many  detonating  solids,  in  which  a  disturbance  of  atom- 
equilibrium  at  the  superficies  of  the  molecule  causes  its  complete 
disruption. 
Now  supposing  two  substances,  A  and  B,  are  in  superficial 
contact  with  one  another  under  such  conditions  that  not  only 
the  superficial  movement  of  the  constituent  parts,  but  also  of  the 
molecule  as  a  whole,  is  disturbed ;  then  three  distinct  cases  are 
possible :. 
Firstly. — The  atoms  change  their  kind  of  motion  without  returning 
to  their  former  state  of  equilibrium ;  no  chemical  change  ensues,  but 
merely  a  physical  phenomenon,  such  as  alteration  of  electric  poten- 
tial, condensation  of  gases,  whether  by  a  cold  surface  or  by  porous 
substances. 
Secondly. — The  internal  movement  of  the  molecule  is  altered,  so 
that  molecules  of  a  different  kind  are  formed  by  combination,  replace- 
