Am.  Jour.  Pharm.) 
May,  1920.  j 
Atoms  and  Chemical  Valence. 
315 
in  the  drawing.  For  a  single  roaster,  it  is  likely  that  a  collector 
6  feet  in  height  will  completely  collect  the  soot. 
To  the  writer's  knowledge  the  collection  of  coffee  soot  is  not 
being  practiced  in  this  country,  but  is  in  Continental  European 
countries  with  reputed  satisfactory  returns.  The  Continental 
preference  for  more  thoroughly  roasted  coffee  may  be  a  factor  in 
this  respect,  because  the  soot  would  consequently  be  richer  in  caffeine. 
Whether  coffee  soot  can  be  established  as  a  profitable  source  of 
caffeine  in  this  country  is  problematical  and  is  a  challenge  to  our  best 
efforts. 
THE  STRUCTURE  OF  ATOMS  AND  ITS  BEARING  ON 
CHEMICAL  VALENCE.* 
By  Irving  Langmuir, 
general  electric  co.,  schenectady,  n.  y. 
According  to  the  well-established  Rutherford-Bohr  theory,  all 
the  positive  electricity  in  an  atom  is  concentrated  in  a  nucleus 
at  its  center.  The  dimensions  of  this  nucleus  are  negligibly  small 
compared  with  those  of  the  rest  of  the  atom,  its  diameter  being  of 
the  order  of  0.00001  of  that  of  the  atom.  The  charge  on  the  nucleus 
is  an  integral  multiple  of  the  charge  of  an  electron  but,  of  course, 
opposite  in  sign.  The  remainder  of  ths  atom  consists  of  electrons 
arranged  in  space  about  the  nucleus,  the  normal  number  of  such 
electrons  (called  the  atomic  number)  being  equal  to  the  number  of 
unit  positive  charges  on  the  nucleus,  so  that  the  atom  as  a  whole  is 
electrically  neutral.  If  the  number  of  electrons  in  the  atom  exceeds 
the  atomic  number  we  have  a  negatively  charged  atom  or  ion,  while 
in  the  reverse  case  a  positively  charged  atom  or  ion  results.  The 
atomic  number  of  any  element  has  been  found  to  be  equal  to  the 
ordinal  number  of  the  element  in  the  periodic  table.  Thus  hydro- 
gen has  the  atomic  number  i,  helium  2,  lithium  3,  carbon  6, 
neon  10,  chlorine  17,  nickel  28,  silver  47,  cerium  58,  tungsten  74, 
radium  88,  and  uranium  92  The  atomic  numbers  can  be  deter- 
mined experimentally  from  the  X-ray  spectrum  so  that  we  are  not 
dependent  upon  the  periodic  table  for  our  knowledge  of  these  num- 
bers. 
*  From  The  Jour,  of  Indus,  and  Engineering  Chem.,  April,  1920. 
