﻿232 
  Scientific 
  Intelligence. 
  

  

  absorbed. 
  A 
  streak 
  of 
  azure-blue 
  water 
  outside 
  the 
  green 
  grotto 
  

   showed 
  the 
  absorption 
  bands 
  previously 
  observed. 
  The 
  red 
  

   grotto 
  showed 
  no 
  trace 
  of 
  red 
  light. 
  The 
  spectroscope 
  showed 
  

   merely 
  the 
  absorption 
  appearances 
  of 
  ordinary 
  sea 
  water. 
  An 
  

   examination 
  of 
  the 
  Swiss 
  ice 
  holes 
  by 
  the 
  spectroscope 
  showed 
  

   an 
  absorption 
  of 
  the 
  red 
  end 
  of 
  the 
  spectrum 
  which 
  gradually 
  

   diminished 
  toward 
  the 
  yellow. 
  No 
  especial 
  or 
  marked 
  absorp- 
  

   tion 
  bands 
  were 
  noticed. 
  The 
  Yellowstone 
  Springs 
  showed 
  in 
  a 
  

   pronounced 
  manner 
  Schonn's 
  aqueous 
  bands 
  in 
  the 
  red 
  and 
  red 
  

   yellow. 
  — 
  Ann. 
  der 
  Physik 
  und 
  Ohemie, 
  No. 
  I, 
  1895, 
  p. 
  175-177. 
  

  

  J. 
  T. 
  

  

  7. 
  The 
  minimum 
  temperature 
  of 
  visibility. 
  — 
  A 
  recent 
  paper 
  by 
  

   P. 
  L. 
  Gray 
  describes 
  experiments 
  made 
  upon 
  a 
  strip 
  of 
  plati- 
  

   num 
  with 
  the 
  object 
  of 
  determining 
  the 
  minimum 
  temperature 
  at 
  

   which 
  it 
  becomes 
  visible 
  in 
  the 
  dark. 
  The 
  author 
  refers 
  to 
  the 
  

   paper 
  by 
  Draper* 
  as 
  giving 
  the 
  only 
  exact 
  results 
  upon 
  the 
  sub- 
  

   ject. 
  He 
  shows 
  that 
  Draper's 
  temperature 
  of 
  minimum 
  visibility, 
  

   corrected 
  by 
  recent 
  determinations 
  of 
  the 
  coefficient 
  of 
  expansion 
  

   of 
  platinum, 
  becomes 
  490° 
  C, 
  instead 
  of 
  525°, 
  and 
  is 
  not 
  very 
  

   much 
  above 
  his 
  own 
  determination 
  given 
  below. 
  Furthermore 
  

   Draper's 
  conclusion 
  that 
  all 
  solid 
  bodies 
  become 
  visible 
  at 
  the 
  same 
  

   temperature 
  is 
  fully 
  confirmed 
  by 
  the 
  author's 
  observations 
  with 
  

   bright 
  and 
  lamp-blacked 
  platinum. 
  

  

  In 
  order 
  to 
  determine 
  the 
  temperature 
  of 
  the 
  platinum 
  strip 
  

   Gray 
  used 
  a 
  modified 
  form 
  of 
  Joly's 
  meldometer,f 
  consisting 
  

   essentially 
  of 
  a 
  strip 
  of 
  very 
  thin 
  platinum, 
  about 
  10 
  cm 
  long, 
  l 
  cm 
  

   broad 
  and 
  -gV 
  mm 
  thick, 
  placed 
  in 
  a 
  vertical 
  plane. 
  In 
  regard 
  to 
  

   its 
  use 
  the 
  author 
  says 
  : 
  

  

  " 
  It 
  can 
  be 
  heated 
  by 
  an 
  electric 
  current, 
  and 
  its 
  linear 
  expan- 
  

   sion 
  is 
  indicated 
  by 
  an 
  optical 
  method, 
  by 
  which 
  an 
  alteration 
  in 
  

   temperature 
  of 
  1° 
  can 
  easily 
  be 
  noticed. 
  The 
  method 
  of 
  calibra- 
  

   tion 
  is 
  described 
  in 
  Joly's 
  paper, 
  and 
  in 
  that 
  already 
  mentioned, 
  

   so 
  that 
  it 
  is 
  unnecessary 
  to 
  do 
  more 
  than 
  briefly 
  refer 
  to 
  it 
  here. 
  

   Minute 
  fragments 
  of 
  substances 
  of 
  known 
  melting-points 
  are 
  

   placed 
  on 
  the 
  strip 
  and 
  watched 
  through 
  a 
  microscope, 
  while 
  the 
  

   temperature 
  is 
  very 
  slowly 
  and 
  cautiously 
  raised 
  until, 
  in 
  any 
  

   case, 
  melting 
  is 
  seen 
  to 
  take 
  place, 
  when 
  the 
  position 
  of 
  the 
  spot 
  

   of 
  light 
  which 
  indicates 
  the 
  expansion 
  of 
  the 
  strip 
  is 
  noted. 
  In 
  

   these 
  experiments 
  the 
  substances 
  used 
  were 
  K 
  2 
  N0 
  3 
  (339°), 
  AgCl 
  

   (451°), 
  KBr 
  (699°), 
  and 
  gold 
  (1041°). 
  From 
  these 
  observations 
  a 
  

   curve 
  showing 
  the 
  relation 
  between 
  temperature 
  and 
  scale-read- 
  

   ings 
  is 
  obtained." 
  .... 
  » 
  

  

  " 
  Method 
  of 
  Making 
  the 
  Experiments. 
  — 
  The 
  first 
  requisite 
  was 
  

   to 
  get 
  the 
  strip 
  in 
  a 
  perfectly 
  dark 
  enclosure, 
  within 
  which 
  both 
  

   eyes 
  could 
  be 
  directed 
  towards 
  it 
  without 
  strain. 
  To 
  this 
  end 
  the 
  

   apparatus 
  was 
  enclosed 
  in 
  a 
  wooden 
  box 
  (blackened 
  within), 
  one 
  

   end 
  of 
  which 
  was 
  replaced 
  by 
  a 
  black 
  velvet 
  cloth, 
  under 
  which 
  

   the 
  observer 
  placed 
  his 
  head, 
  and 
  which 
  he 
  could 
  gather 
  round 
  

  

  *On 
  the 
  production 
  of 
  light 
  by 
  heat, 
  Phil. 
  Mag., 
  xxx, 
  345, 
  1847. 
  

   f 
  Proc. 
  R. 
  Irish 
  Acad 
  , 
  III, 
  ii, 
  38, 
  1891-92. 
  

  

  