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OCCLUSION  OF  HYDROGEN  BY  METALS. 
OCCLUSION  OF  HYDROGEN  BY  METALS. 
The  master  of  the  mint  has  applied  this  term  to  the  absorption 
of  gases  by  what  he  terms  colloid  metals. 
A  new  method  of  charging  the  metals  with  hydrogen  at  low 
temperatures  has  lately  been  discovered  by  him. 
When  a  plate  of  zinc  is  placed  in  diluted  sulphuric  acid  hy- 
drogen gas  is  freely  evolved  from  the  surface  of  the  metal ;  but 
no  hydrogen  is  occluded  and  retained.  A  negative  result  was,  in- 
deed, to  be  expected  from  the  crystalline  structure  of  zinc.  But 
a  thin  plate  of  palladium  in  the  same  acid,  and  brought  into  con- 
tact with  the  zinc,  soon  becomes  largely  charged  with  the  hy- 
drogen, which  is  transferred  to  its  surface.  The  charge  taken 
up  in  an  hour  by  a  palladium  plate  amounted  to  173  times  its 
volume. 
Although  the  hydrogen  enters  the  palladium,  and  no  doubt 
pervades  the  whole  mass  of  the  metal,  it  exhibits  no  disposition 
to  leave  that  substance  even  in  a  vacuum  at  the  temperature  of 
its  absorption.  Occluded  hydrogen  is  therefore  no  longer  a  gas, 
whatever  may  be  thought  of  its  physical  condition.  When  palla- 
dium charged  with  hydrogen  is  left  exposed  to  the  atmosphere, 
the  metal  is  apt  to  become  suddenly  hot,  and  to  lose  its  gas  en- 
tirely by  spontaneous  oxidation. 
The  condition  of  hydrogen,  as  occluded  by  a  colloid  metal, 
may  be  studied  with  most  advantage  in  its  union  with  palladium, 
where  the  proportion  of  gas  held  is  considerable.  The  largest 
absorption  of  hydrogen  observed  was  in  the  case  of  palladium 
thrown  down  upon  a  thin  platinum  wire  by  electric  deposition. 
Such  a  specimen  of  metal  occluded  982  times  its  volume  of  hy- 
drogen, or  by  weight — 
Palladium,  99.277 
Hydrogen,  .723 
100. 
or  an  approximation  to  the  compound  Pd  H. 
Professor  Graham  thinks  that  the  passage  of  hydrogen  through 
metals  is  always  preceded  by  the  condensation,  or  occlusion  of 
the  gas.  The  "  solution  affinity  "  of  metals  appears  to  be  nearly 
confiiied  to  hydrogen  and  carbonic  oxide  ;  metals  are  not  sensibly 
penetrated  by  other  gases  than  these. — The  Med,  News  and 
Library^  Jan.  1869,  from  Med.  Press  and  Circular,  Sept.  2, 
1868. 
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