﻿Fenner 
  — 
  Stability 
  Relations 
  of 
  Silica 
  Minerals. 
  361 
  

  

  The 
  inversion 
  from 
  a- 
  to 
  /3-cristobalite 
  which 
  occurs 
  upon 
  

   heating 
  is 
  perfectly 
  sharp 
  but 
  is 
  accompanied 
  by 
  a 
  very 
  small 
  

   energy-change. 
  Therefore 
  the 
  heating-curve 
  method 
  ordinarily 
  

   applicable 
  for 
  the 
  determination 
  of 
  a 
  sharply 
  denned 
  transition- 
  

   point 
  is 
  not 
  adapted 
  for 
  this 
  particular 
  case. 
  By 
  means 
  of 
  a 
  

   simple 
  modification,* 
  however, 
  very 
  precise 
  measurements 
  may 
  

   be 
  made. 
  The 
  essential 
  feature 
  of 
  the 
  appliance 
  consisted 
  of 
  the 
  

   use 
  of 
  two 
  thermocouples, 
  one 
  of 
  which 
  was 
  imbedded 
  in 
  the 
  

   substance 
  in 
  question 
  and 
  gave 
  directly 
  its 
  temperature 
  in 
  terms 
  

   of 
  electromotive 
  force, 
  and 
  the 
  second 
  was 
  imbedded 
  in 
  a 
  neu- 
  

   tral 
  body 
  which 
  was 
  exposed 
  to 
  the 
  same 
  temperature-treatment 
  

   as 
  the 
  first, 
  but 
  underwent 
  no 
  transformation 
  involving 
  an 
  

  

  Fig. 
  5. 
  

  

  absorption 
  of 
  heat 
  within 
  the 
  temperature 
  range 
  under 
  obser- 
  

   vation. 
  The 
  two 
  thermocouples 
  were 
  connected 
  in 
  such 
  a 
  

   manner 
  that 
  the 
  electromotive 
  force 
  due 
  to 
  the 
  temperature 
  of 
  

   one 
  charge 
  was 
  opposed 
  by 
  that 
  of 
  the 
  second, 
  so 
  that 
  readings 
  

   expressed 
  differences 
  in 
  temperature 
  between 
  the 
  two. 
  The 
  

   general 
  arrangement 
  is 
  shown 
  in 
  fig. 
  5. 
  The 
  materials 
  of 
  the 
  

   thermoelement 
  wires 
  were 
  pure 
  copper 
  and 
  the 
  alloy 
  constan- 
  

   tan, 
  whose 
  electromotive 
  force 
  at 
  different 
  temperatures 
  up 
  to 
  

   360° 
  has 
  been 
  determined 
  by 
  Adams 
  and 
  Johnston. 
  f 
  These 
  

   metals 
  give 
  a 
  much 
  larger 
  electromotive 
  force 
  than 
  Pt 
  against 
  

   Pt-Ph, 
  but 
  can 
  only 
  be 
  used 
  for 
  rather 
  low 
  temperatures. 
  

  

  In 
  the 
  arrangement 
  shown 
  in 
  fig. 
  5 
  the 
  element 
  A 
  is 
  

   placed 
  within 
  the 
  charge 
  under 
  investigation, 
  and 
  the 
  ele- 
  

  

  *Due 
  to 
  W. 
  Roberts-Austen. 
  See 
  G. 
  K. 
  Burgess, 
  Bull. 
  Bureau 
  Standards, 
  

   v, 
  p. 
  210, 
  1908-9. 
  

  

  f 
  L. 
  H. 
  Adams 
  and 
  J. 
  Johnston, 
  this 
  Journal, 
  (4), 
  xxxiii, 
  534, 
  1912. 
  The 
  

   constantan 
  wire 
  used 
  in 
  making 
  my 
  own 
  thermoelements 
  was 
  from 
  the 
  same 
  

   bobbin 
  as 
  that 
  used 
  by 
  Adams 
  and 
  Johnston 
  for 
  the 
  calibration 
  curve 
  above 
  

   published. 
  

  

  