﻿216 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  194 
  3 
  

  

  Melting 
  in 
  a 
  vacuum, 
  out 
  of 
  contact 
  with 
  furnace 
  gases, 
  gives 
  a 
  means 
  

   of 
  preparing 
  many 
  substances 
  in 
  a 
  state 
  of 
  great 
  purity 
  and 
  sound- 
  

   ness, 
  unobtainable 
  in 
  other 
  ways, 
  and 
  is 
  already 
  applied 
  to 
  certain 
  

   alloys 
  in 
  amounts 
  of 
  several 
  tons 
  at 
  a 
  time. 
  

  

  Many 
  chemical 
  reactions 
  are 
  made 
  possible 
  or 
  are 
  accelerated 
  by 
  

   high 
  pressures. 
  The 
  autoclave 
  has 
  long 
  been 
  a 
  familiar 
  piece 
  of 
  

   chemical 
  plant, 
  but 
  some 
  of 
  the 
  processes 
  for 
  the 
  production 
  of 
  am- 
  

   monia 
  from 
  the 
  air 
  or 
  for 
  the 
  hydrogenation 
  of 
  coal 
  to 
  form 
  oils 
  

   and 
  petrol 
  called 
  for 
  higher 
  pressures 
  than 
  those 
  of 
  the 
  usual 
  auto- 
  

   clave, 
  and 
  recent 
  work, 
  especially 
  in 
  the 
  United 
  States, 
  has 
  shown 
  

   the 
  remarkable 
  results 
  which 
  may 
  be 
  obtained 
  under 
  pressures 
  of 
  

   many 
  thousands 
  of 
  atmospheres. 
  To 
  construct 
  vessels 
  to 
  be 
  oper- 
  

   ated 
  at 
  such 
  pressures, 
  sometimes 
  combined 
  with 
  high 
  temperatures, 
  

   naturally 
  involves 
  entirely 
  new 
  engineering 
  methods. 
  The 
  materials 
  

   used 
  are 
  mainly 
  steels, 
  although 
  hard 
  carbides 
  may 
  have 
  to 
  be 
  used 
  

   for 
  certain 
  parts; 
  but 
  for 
  the 
  highest 
  pressures 
  counterbalancing 
  

   stresses 
  have 
  to 
  be 
  applied 
  by 
  shrinking 
  one 
  cylinder 
  over 
  another 
  or 
  

   in 
  various 
  ways 
  producing 
  an 
  internal 
  stress 
  opposite 
  to 
  that 
  which 
  

   will 
  arise 
  in 
  operation. 
  Here 
  is 
  another 
  new 
  branch 
  of 
  engineering 
  

   of 
  great 
  scientific 
  interest, 
  extending 
  the 
  range 
  of 
  usefulness 
  of 
  known 
  

   materials. 
  

  

  The 
  substitution 
  of 
  one 
  metal 
  for 
  another 
  for 
  particular 
  purposes 
  

   is 
  not 
  always 
  due 
  to 
  inherent 
  advantages, 
  but 
  is 
  often 
  a 
  consequence 
  

   of 
  a 
  policy 
  of 
  self-sufficiency 
  adopted 
  by 
  an 
  industrial 
  country. 
  This 
  

   motive 
  has 
  been 
  very 
  prominent 
  in 
  recent 
  years. 
  It 
  is 
  natural 
  that 
  

   Germany, 
  producing 
  much 
  aluminum 
  but 
  very 
  little 
  copper, 
  should 
  

   adopt 
  the 
  lighter 
  metal 
  for 
  overhead 
  electric 
  power 
  cables, 
  but 
  a 
  

   similar 
  replacement 
  in 
  the 
  windings 
  of 
  motors 
  and 
  dynamos 
  was 
  

   purely 
  due 
  to 
  conditions 
  of 
  blockade 
  and 
  could 
  not 
  be 
  defended 
  on 
  

   other 
  grounds. 
  During 
  the 
  period 
  of 
  armament 
  and 
  of 
  hostilities 
  

   such 
  substitutions 
  have 
  been 
  very 
  numerous, 
  but 
  the 
  subject 
  is 
  too 
  

   big 
  to 
  be 
  entered 
  on 
  here. 
  A 
  few 
  instances 
  of 
  substitution 
  under 
  

   normal 
  conditions 
  may 
  be 
  mentioned. 
  The 
  saving 
  of 
  weight 
  by 
  using 
  

   light 
  alloys 
  of 
  aluminum 
  or 
  magnesium 
  in 
  place 
  of 
  steel 
  has 
  mainly 
  

   been 
  utilized 
  in 
  aircraft 
  and 
  rolling-stock 
  construction, 
  but 
  it 
  has 
  

   occasionally 
  been 
  applied 
  in 
  ordinary 
  structural 
  work. 
  A 
  bridge 
  in 
  

   Pittsburgh, 
  having 
  been 
  condemned 
  as 
  insufficiently 
  strong 
  for 
  the 
  

   increased 
  traffic, 
  was 
  lightened 
  by 
  replacing 
  the 
  wrought-iron 
  road 
  

   girders 
  and 
  floor 
  by 
  aluminum 
  alloy. 
  The 
  main 
  girders 
  were 
  still 
  in 
  

   good 
  condition, 
  and 
  the 
  lightening 
  of 
  the 
  dead 
  load 
  gave 
  the 
  bridge 
  

   a 
  new 
  lease 
  of 
  life. 
  

  

  The 
  substitution 
  of 
  one 
  material 
  for 
  another 
  is 
  not, 
  as 
  a 
  rule, 
  a 
  

   simple 
  matter. 
  Metals 
  differ 
  not 
  only 
  in 
  strength 
  but 
  also 
  in 
  elastic 
  

   properties, 
  and 
  this 
  difference 
  has 
  to 
  be 
  allowed 
  for. 
  The 
  new 
  Quebec 
  

  

  