OVERHEATED  STEAM  IN  CARBONIZING  OF  WOOD. 
555 
only  for  the  finest  powder,  is  in  general  use  for  the  cheaper 
kinds,  so  simple  and  certain  is  its  preparation  by  means  of 
steam. 
M.  Violette  communicates  now  to  the  Academy  some  new 
results.  He  shows  that  the  change  to  charcoal  takes  place  dif- 
ferently with  different  woods,  and  that  the  products  of  the  same 
temperature  differ  in  elementary  constitution.  Exposed  to  moist 
air,  the  charcoals  absorb  more  water,  the  lower  the  temperature 
to  which  they  were  exposed,  and  the  inverse  is  true  of  their 
power  of  conducting  heat  or  electricity.  Charcoal  made  at 
1500°  C.  conducts  much  better  than  the  charcoal  of  gas  retorts, 
and  serves  perfectly  for  electric  illumination.  The  density  in- 
creases in  the  same  proportion.  When  lighted,  charcoals  remain 
ignited  for  a  time,  which  decreases  as  the  temperature  of  car- 
bonization increases.  The  charcoal  made  at  260°  C.  burns  more 
easily  and  longer;  that  made  between  1000°  and  1500°  C.  will 
not  ignite  or  burn. 
The  most  inflammable  of  all  charcoals  is  that  of  an  Agaricus. 
It  takes  fire  spontaneously  at  300°  C.  Other  charcoals,  pre- 
pared at  the  temperature  300°  C,  take  fire  in  air  spontaneously 
between  360°  and  380°,  according  to  the  wood  that  has  afforded 
them,  the  charcoals  of  the  lighter  woods  burning  the  most 
readily. 
When  charcoals  are  mixed  with  sulphur,  they  inflame  at  a 
temperature  much  below  that  required  when  alone ;  the  mixture 
of  the  two  prepared  between  150°  and  400°  C,  is  wholly  con- 
sumed at  250Q  C.  On  the  contrary,  when  the  charcoal  employed 
has  been  prepared  at  1000°  or  ]500°  C,  only  the  sulphur  burns. 
To  decompose  saltpeter,  the  charcoals  require  a  higher  tem- 
perature ;  a  heat  of  400°  C.  is  needed  for  charcoals  prepared 
between  150°  and  432°,  and  a  red  heat  for  those  made  between 
1000°  and  1500°. 
Sulphur  decomposes  saltpeter  at  a  higher  temperature  than 
charcoal  requires,  viz.,  at  432°.  The  sulphur  alone  inflames  in 
common  air  at  250°  C,  and  not  at  150°,  as  stated  in  treatises  on 
chemistry. 
The  deflagration  of  powder  takes  place  at  250°  but  its  com- 
bustibility varies  with  the  charge  and  the  size  of  the  grain.  The 
