﻿528 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  Organic 
  matter 
  may 
  also 
  mask 
  the 
  presence 
  of 
  iron, 
  so 
  that 
  the 
  

   clay, 
  instead 
  of 
  burning 
  white, 
  would 
  burn 
  red 
  at 
  a 
  temperature 
  

   above 
  that 
  at 
  which 
  the 
  organic 
  matter 
  passes 
  off. 
  Below 
  that 
  

   temperature, 
  though, 
  the 
  vegetable 
  matter 
  would 
  tend 
  to 
  keep 
  the 
  

   iron 
  reduced, 
  and 
  the 
  color 
  would 
  be 
  gray 
  instead 
  of 
  red. 
  

  

  In 
  most 
  chemical 
  analyses 
  the 
  organic 
  matter 
  is 
  seldom 
  de- 
  

   termined 
  separately, 
  but 
  the 
  amount 
  of 
  it 
  can 
  sometimes 
  be 
  judged 
  

   from 
  the 
  ratio 
  between 
  the 
  loss 
  on 
  ignition 
  and 
  amount 
  of 
  alumina 
  

   in 
  the 
  clay. 
  

  

  Organic 
  matter 
  may 
  increase 
  the 
  plasticity 
  of 
  a 
  clay 
  provided 
  too 
  

   much 
  sand 
  i^ 
  not 
  present, 
  in 
  which 
  case 
  a 
  highly 
  carbonaceous 
  clay 
  

   might 
  be 
  very 
  lean, 
  {see 
  " 
  Plasticity 
  of 
  clays 
  ", 
  p. 
  539) 
  

  

  Wate7' 
  in 
  clay 
  

  

  All 
  clays 
  contain 
  two 
  kinds 
  of 
  water: 
  

  

  1 
  Hygroscopic 
  water, 
  or 
  moisture 
  

  

  2 
  Chemically 
  combined 
  water 
  

  

  Moisture. 
  Clays 
  contain 
  two 
  kinds 
  of 
  moisture 
  : 
  

  

  1 
  That 
  which 
  is 
  held 
  in 
  the 
  pores 
  of 
  the 
  clay 
  by 
  capillary 
  at- 
  

   traction. 
  

  

  2 
  That 
  which 
  adheres 
  to 
  the 
  surface 
  of 
  each 
  clay 
  grain 
  as 
  a 
  thin 
  

   film. 
  

  

  The 
  latter 
  is 
  of 
  little 
  importance 
  practically. 
  

  

  The 
  former 
  is 
  of 
  importance 
  in 
  connection 
  with 
  the 
  shrinkage 
  

   and 
  plasticity 
  of 
  clays. 
  The 
  amount 
  of 
  total 
  moisture 
  contained 
  in 
  

   clays 
  varies 
  within 
  wide 
  limits. 
  In 
  some 
  air-dried 
  clays 
  it 
  may 
  be 
  

   as 
  low 
  as 
  .5^, 
  while 
  in 
  those 
  freshly 
  taken 
  from, 
  the 
  bank 
  it 
  may 
  

   reach 
  30^ 
  or 
  40^. 
  Capillary 
  moisture 
  is 
  absorbed 
  by 
  clays 
  only 
  when 
  

   they 
  are 
  brought 
  into 
  actual 
  contact 
  with 
  water, 
  but 
  that 
  which 
  

   forms 
  a 
  film 
  on 
  the 
  surface 
  of 
  the 
  clay 
  particles 
  is 
  readily 
  absorbed 
  

   by 
  the 
  clay 
  from 
  the 
  atmosphere, 
  and 
  to 
  a 
  certain 
  extent 
  given 
  off 
  

   again 
  as 
  readily, 
  so 
  that 
  some 
  days 
  a 
  brick 
  if 
  left 
  exposed 
  to 
  the 
  

   air 
  would 
  weigh 
  more 
  than 
  on 
  others. 
  The 
  amount 
  of 
  either 
  kind 
  

  

  