ACTION  OF  WATER  UPON  GLASS. 
07 
rent  of  carbonic  acid  is  passed,  absorbs  this  gas  in  a  few  moments, 
and  afterwards  effervesces  briskly  with  acids. 
6.  Powdered  glass,  boiled  for  several  hours  with  sulphate  of 
lime,  produces  a  considerable  quantity  of  sulphate  of  soda.  This 
reaction  explains  why  the  walls  and  floors  of  workshops,  in  which 
plate-glass  is  polished,  are  always  covered  with  an  efflorescence 
of  sulphate  of  soda.  The  plaster  which  serves  to  fix  the  glass 
furnishes  the  sulphuric  acid,  and  the  glass  the  soda,  of  which 
this  salt  is  composed. 
7.  All  glasses  reduced  to  fine  powder  immediately  restore  the 
blue  color  of  red  litmus-paper  and  tincture,  and  change  syrup 
of  violets  to  green.  Powdered  glass  which  has  undergone  the 
action  of  cold  water,  continues  to  alter  in  boiling  water. 
8.  Crystal-glass  in  fine  powder,  agitated  for  a  few  minutes 
with  water  containing  a  very  small  quantity  of  acid,  gives  a 
black  deposit  of  sulphuret  of  lead  by  treatment  with  sulphuretted 
hydrogen.  After  half  an  hour's  ebullition  with  water  and  the 
addition  of  an  acid,  5  grms.  of  crystal  in  powder  furnished  0-050 
of  sulphuret  of  lead,  representing  a  decomposition  of  about  8  per 
cent.  Flint-glass,  which  contains  still  more  oxide  of  lead,  under- 
goes a  still  more  considerable  decomposition. 
Devitrified  glass  behaves  like  ordinary  glass  with  water,  ex- 
cept that  it  appears  to  be  still  more  easily  decomposed. 
After  boiling  for  five  days,  a  specimen  of  glass  similar  in  com- 
position to  the  first-mentioned  had  undergone  a  decomposition 
corresponding  to  a  third  of  its  weight,  and  the  silicate  of  soda 
yielded  by  it  had  also  the  formula — 
3(Si03)2ISraO. 
Thus  powdered  glass  is  decomposed  by  contact  with  water  or 
air  with  an  ease  and  rapidity  which  appear  very  extraordinary, 
considering  the  great  stability  of  vessels  formed  of  cast  or  blown 
glass.  Is  the  surface  of  the  glass  in  this  form  in  a  particular 
state,  which  modifies  its  properties  ?  This  does  not  appear  pro- 
bable when  we  consider  that  plate-glass,  from  the  surface  of 
which  several  millimetres  have  been  removed  in  polishing,  is 
equally,  if  not  more  permanent  in  moist  air  and  water  than  com- 
mon glass,  and  that  in  all  cases  the  crude  plate-glass  presents  neither 
more  nor  less  resistance  to  atmospheric  agents  than  the  others. 
The  author  regards  the  difference  in  action  of  water  upon  glass 
