Intelligence  and  Miscellaneous  Articles.  475 
Protobromide  of  iodine  is  a  solid  which  may  be  obtained  crystal- 
lized by  sublimation.  At  the  ordinary  temperature  it  emits  a 
vapour  a  slight  thickness  of  which  is  of  a  copper-red  colour,  while 
80  centims.  thickness  has  a  currant-red  tint.  The  absorption- 
spectrum  of  this  vapour,  of  the  same  kind  as  those  of  iodine  and 
bromine,  consists  of  very  fine  lines  situated  in  the  red,  the  yellow, 
and  the  orange.  It  differs  from  the  effect  observed  when  the  light 
is  passed  through  successive  layers  of  the  vapours  of  iodine  and 
bromine. 
Volatile  organic  matters  can  give,  like  the  other  vapours,  ab- 
sorption-spectra. Thus  dry  alizarine,  heated  with  care,  emits  vapours 
which  produce,  in  the  middle  region  of  the  spectrum,  systems  of 
sensibly  equidistant  lines. —  Comptcs  Rendus  de  I'Acad.  des  Sciencest 
April  29,  1872,  p.  1190. 
DEMAGNETIZATION     OF    ELECTROMAGNETS.       BY    ROBERT    W. 
WILLSONj  JUNIOR  CLASS,  HARV.  COLL. 
Wiedemann  has  shown  (Pogg.  Ann.  vol.  c.  p.  235,  Ann.  de  Min. 
(3)  vol.  1.  p.  189)  that  the  intensity  of  the  current  necessary  to 
demagnetize  a  steel  magnet  is  much  less  than  that  of  the  current 
by  which  the  bar  was  originally  magnetized. 
It  has  occurred  to  me  to  experiment  with  a  view  to  ascertaining 
how  far  this  principle  can  be  applied  to  electromagnets. 
The  apparatus  used  consisted  of  a  cylindrical  bar  of  soft  iron 
8  centims.  in  length  and  1  centim.  in  diameter,  slightly  rounded  at 
the  end.  The  armature  was  a  piece  of  soft  iron  of  the  same  diameter 
and  2  centims.  in  length. 
Around  this  core  were  placed  two  concentric  helices,  wound  in 
opposite  directions,  through  each  of  which  could  be  passed  the  cur- 
rent of  a  single  Grove's  cell. 
The  method  of  experimenting  was  as  follows.  A  current  was 
first  passed  through  the  inner  helix,  which,  for  convenience,  I  shall 
call  A,  and  the  weight  supported  by  the  bar  was  noted.  This  cur- 
rent being  broken,  the  outer  helix,  B,  was  introduced  into  the  cir- 
cuit, and  the  corresponding  weight  noted.  The  current  through  B 
being  then  broken,  and  that  through  A  closed,  after  a  short  time  the 
current  was  again  passed  through  B  and  the  weight  noted.  The 
following  Table  shows  the  results  : — 
Wt.  supported  by 
IT- 
iT 
A  diminished  by  B 
490  grms. 
250 
130 
470 
240 
130 
460 
230 
130 
460 
220 
120 
420 
220 
130 
410 
200 
110 
Mean    .  .    452  227  125 
Taking  the  mean  result,  we  see  that  while  the  helix  B  can  only 
develope  sufficient  magnetism  in  the  bar  to  render  it  capable  of  sus- 
