﻿L. 
  V. 
  Pirsson 
  — 
  Mineralogical 
  Notes. 
  

  

  405 
  

  

  a 
  reddish 
  translucent 
  sublimate. 
  In 
  the 
  open 
  tube 
  with 
  a 
  good 
  

   current 
  of 
  air 
  volatilizes 
  and 
  deposits 
  in 
  the 
  upper 
  part 
  small 
  

   glittering 
  octahedrons, 
  which 
  under 
  the 
  microscope 
  in 
  polar- 
  

   ized 
  light 
  proved 
  isometric 
  (As 
  2 
  O 
  s 
  ). 
  Before 
  the 
  blowpipe 
  on 
  

   charcoal 
  gives 
  characteristic 
  odors 
  for 
  sulphur 
  and 
  arsenic. 
  

   Fused 
  with 
  carbonate 
  of 
  soda 
  gives 
  the 
  reactions 
  on 
  dissolving 
  

   for 
  sulphur 
  and 
  arsenic. 
  

  

  The 
  association 
  of 
  the 
  realgar 
  and 
  orpiment 
  is 
  such 
  that 
  no 
  

   definite 
  statement 
  can 
  be 
  made 
  as 
  to 
  which 
  forms 
  first, 
  but 
  

   realgar 
  is 
  certainly 
  the 
  last 
  formed 
  upon 
  many 
  of 
  the 
  speci- 
  

   mens. 
  Whether 
  it 
  is 
  formed 
  through 
  a 
  conversion 
  of 
  the 
  

   orpiment 
  or 
  as 
  a 
  separate 
  deposition 
  is 
  uncertain, 
  but 
  the 
  

   specimens 
  seem 
  to 
  indicate 
  that 
  the 
  latter 
  is 
  the 
  case. 
  Sili- 
  

   ceous 
  sinter 
  is 
  the 
  only 
  other 
  mineral 
  occurring 
  with 
  these 
  

   arsenical 
  sulphides. 
  

  

  Art. 
  XLII. 
  — 
  Mineralogical 
  Notes 
  ; 
  by 
  L. 
  Y. 
  Pirsson. 
  

  

  1. 
  Cerussite. 
  — 
  Some 
  specimens 
  of 
  cerussite, 
  obtained 
  through 
  

   Messrs. 
  English 
  & 
  Co., 
  of 
  New 
  York, 
  from 
  the 
  Red 
  Cloud 
  

   Mine 
  in 
  Yuma 
  Co., 
  Arizona, 
  contain 
  twin 
  crystals 
  in 
  which 
  

   the 
  twinning 
  plane 
  is 
  the 
  uncommon 
  form 
  i-S, 
  130. 
  Since, 
  

   moreover, 
  they 
  show 
  some 
  unusual 
  developments 
  in 
  their 
  crys- 
  

   tal 
  form, 
  it 
  has 
  been 
  thought 
  that 
  a 
  description 
  of 
  them 
  would 
  

   be 
  of 
  interest. 
  The 
  specimens 
  are 
  in 
  the 
  cabinet 
  of 
  Prof. 
  

   George 
  J. 
  Brush. 
  

  

  The 
  greater 
  number 
  of 
  those 
  observed 
  have 
  the 
  form 
  shown 
  

   in 
  fig. 
  1, 
  which 
  presents 
  them 
  in 
  a 
  basal 
  projection. 
  This 
  

  

  gives 
  a 
  much 
  better 
  idea 
  of 
  these 
  crystals, 
  shaped 
  like 
  arrow- 
  

   heads, 
  than 
  an 
  orthographic 
  projection. 
  They 
  are 
  of 
  various 
  

   sizes 
  up 
  to 
  half 
  an 
  inch 
  in 
  length, 
  the 
  largest 
  observed. 
  They 
  

  

  