32 
Report  on  Atomic  Weights. 
Am.  Jour.  Pharm. 
January,  1920. 
foss^  determined  thfe  density  of  acetylene,  ethane  and  ethylene. 
Acetylene  proved  to  be  unsatisfactory,  because  of  its  tendency  to 
polymerize.  From  ethane  he  obtained  the  value  C  =  12.006, 
and  from  ethylene  C  =  12.004.  On  account  of  some  uncertainties 
in  the  reduction,  he  prefers,  provisionally,  the  value  C  =  12.00. 
Secondly,  Batuecas^  determined  the  density  of  ethane,  and  re- 
duced his  observations  by  3  methods,  giving  C  =  12.005,  ^  1-999 
and  1 1.996.  The  last  two  being  concordant  he  regards  as  preferable, 
and  their  mean,  C  =  11.998,  he  adopts.  It  will  be  remembered 
that  Richards  and  Hoover,  by  purely  chemical  methods,  found  C  = 
12.005;  and  a  later  combination  of  all  determinations  published 
before  19 18  gave  the  chairman  of  the  committee  the  mean  value 
C  =  12.0025.  For  ordinary  purposes  the  rounded-off  value  C  = 
12.00  may  be  used,  and  is  so  given  in  the  table. 
Bromine. — Three  sets  of  determinations  of  the  molecular  weight 
of  hydrogen  bromide  have  been  made  in  Guye's  laboratory  at  Geneva, 
by  Moles, ^  Reiman,^  and  Murray.^  The  acid  used  was  prepared 
by  several  distinct  methods,  and  all  gave  concordant  results,  which 
may  be  summarized  as  follows,  when  H  =  1.0078: 
These  values  are  wonderfully  concordant  and  the  variations  are 
far  within  the  allowable  limits  of  experimental  error.  In  a  recent 
combination,  by  the  chairman  of  this  committee,  of  all  the  available 
data  relative  to  the  atomic  weight  of  bromine,  the  value  found  was 
Br  =  79.9228,  in  complete  harmony  with  the  Geneva  determina- 
tions.   For  ordinary  purposes  the  rounded-off  figure  79.92  is  enough. 
Boron  and  Fluorine • — In  a  very  original  investigation  Smith  and 
Van  Haagen^^  have  simultaneously  redetermined  the  atomic  weights 
of  boron  and  fluorine.    Their  starting  point  was  anhydrous  borax, 
^  /.  chim.  phys.,  16:  175,  1918. 
6  Ibid.,  16:  322,  1918. 
^  Ibid.,  14:  389,   1916.    See  review  by  Guye  ill  the  same  number,  p.  361.' 
8  Ibid.,  15 :  293,  1917. 
^  Ibid.,  15:  334,  1917.  Reiman  and  Murray  assume  H  =  1.008;  Moles 
prefers  1.0076. 
1°  Carnegie  Inst.  Pub.,  1918,  p.  267. 
Mol.  Wt.  HBr. 
At.  Wt.  Br. 
Moles . . 
Reiman 
Murray 
80.9332 
90.932 
80.930 
79  9254 
79.9242 
79.9222 
