174  Soluble  Qlass  and  its  Applications.      j A  Aprin,  i872.KM 
detect  this  and  similar  impositions  is  simply  to  digest  a  sample  of  the 
suspected  substance  in  ether  or  absolute  alcohol,  when  the  coloring 
matter  will  be  dissolved  with  ease,  and  the  sugar  crystals,  or  wood 
fibre  (which  is  also  used  for  dishonest  purposes)  will  remain  undis- 
solved.— Franklin  Inst.  Journ.,  March,  1872. 
SOLUBLE  GLASS  AND  ITS  APPLICATIONS. 
The  recent  epidemic  of  fires,  if  we  may  be  allowed  the  expression, 
has  in  many  cases  called  the  attention  of  the  public  to  this  remark- 
able compound,  and  stimulated  inquiry  as  to  its  uses  and  the  methods 
of  applying  it.  Although  it  has  been  known  for  many  years,  it  has 
thus  far  failed  to  come  into  very  general  use,  partly  because  the  pub- 
lic have  expected  from  it  more  than  was  reasonable,  and  have  conse- 
quently been  disappointed  in  practice ;  and  partly  because  the  ex- 
travagant praises  of  some  of  its  friends  have  placed  it  in  the  same 
category  as  quack  medicines  and  other  articles  which  are  said  to  cure 
all  diseases  and  remove  all  difficulties,  and  hence  are  regarded  by  the 
more  sensible  portion  of  the  community  as  not  being  good  for  any- 
thing. In  reality,  however,  it  does  possess  the  most  valuable  proper- 
ties, and  may  be  used  to  great  advantage  under  a  great  variety  of 
circumstances. 
Soluble  glass  is  simply  a  variety  of  purely  alkaline  glass  in  which 
the  alkali  is  in  excess.  Ordinary  window-glass  is  a  compound  of  si- 
lica with  potash  or  soda,  and  in  some  cases  lime ;  oxide  of  lead  added 
to  the  compound  of  silica  and  potash  or  soda  gives  flint  glass  ;  Bohe- 
hemian  glass  is  a  compound  of  silica,  soda  and  lime  ;  and  the  coarse 
glass  used  for  bottles  contains  much  iron  and  some  alumina,  which  is 
the  base  of  clay.  According  to  the  quantity  of  alkali  employed,  the 
glass  will  be  soluble  or  insoluble  ;  it  being  understood  that  all  glass  is 
soluble  to  a  certain  extent.  Old  window-panes  that  have  been  ex- 
posed to  the  elements  for  years,  are  in  general  so  corroded  that  their 
surfaces  are  no  longer  perfectly  transparent ;  and  common  flint  glass, 
when  finely  powdered,  dissolves  in  water  to  such  an  extent  that  its 
presence  can  be  detected  by  the  least  delicate  re-agents.  But  when 
the  proportion  of  alkali  is  largely  imcreased,  and  especially  when  the 
compound  consists  of  pure  alkali  and  pure  silica,  we  obtain  a  glass 
which  dissolves  entirely  in  water,  and  which  may  be  applied  as  an  in- 
combustible varnish  to  wooden  articles,  or  used  as  a  cement  or  as  a 
