524 
ON  HYDRAULIC  LIMES,  ETC. 
1841,  when  these  views  were  first  put  forward,  they  have  been 
adopted  in  practice  to  a  considerable  extent. 
In  the  hardening  of  the  siliceous  limestones,  the  carbonic 
acid  of  the  air  appears  to  separate  a  part  of  the  silica ;  and  the 
portions  which  are  in  contact  with  a  sufficient  quantity  of  car- 
bonate of  lime,  pass  to  the  state  of  silicate  of  lime. 
The  author's  memoir  of  1841  indicated  many  industrial  ap- 
plications of  the  artificial  injection  of  mineral  substances  into 
the  interior  of  porous  bodies,  whether  organic  or  inorganic.  In 
the  present  paper  he  gives  a  new  series  of  observations. 
The  silicatization  of  sculptures  and  buildings  gives  rise  to  co- 
lorations of  the  stone,  which  are  often  very  considerable,  and 
render  the  joints  and  veins  more  distinct.  In  getting  rid  of  this 
inconvenience,  two  essential  points  are  to  be  fulfilled, — chalk 
walls  remain  too  white,  whilst  certain  ferruginous  limestones  ac- 
quire dark  tints.  The  silicatization  of  white  limestones  is  ef- 
fected with  a  double  silicate  of  potash  and  manganese.  This  is  a 
vitreous  substance  of  a  deep  violet  color,  which  furnishes  a  brown 
solution,  and  this  applied  in  silicatization,  deposits  a  little  oxide 
of  manganese  in  the  artificial  siliceous  paste.  Oxide  of  cobalt 
also  combines,  but  in  smaller  quantity,  with  the  silicate  of  pot- 
ash ;  the  silica  precipitated  by  a  current  of  carbonic  acid  is  of  a 
fine  azure-blue,  and  this  silicate  may  be  employed  in  the  treat- 
ment of  white  marbles.  When  the  stones  are  too  dark,  which  is 
the  most  general  case,  excellent  results  may  be  obtained  by  sus- 
pending in  the  solution  of  silicate,  small  quantities  of  artificial 
sulphate  of  baryta,  which,  penetrating  into  the  porous  stone,  at 
the  same  time  with  the  formation  of  the  siliceous  layer,  remains 
there,  entering  into  a  state  of  chemical  combination.  The  join- 
ings may  be  made  with  ordinary  cements,  lightened  in  color  with 
white  substances ;  but  they  may  be  more  completely  concealed 
by  fragments  of  the  stone  itself,  mixed  with  the  vitreous  silicate 
of  potash,  the  whole  very  finely  pulverized,  and  applied  in  the 
form  of  a  liquid  paste. 
Coloring  of  Stone. — Stones  may  be  tinged  of  various  colors 
by  impregnating  them  with  certain  metallic  salts,  and  afterwards 
producing  precipitates  of  colored  compounds.  Thus  by  impreg- 
nating stones  with  salts  of  lead  or  copper,  and  bringing  them  in 
contact  with  sulphuretted  hydrogen  gas  of  hydrosulphate  of  am- 
