VARIETIES. 
85 
glue.  The  tablets  or  slabs  should  remain  in  the  bath  a  sufficient  time 
to  become  thick  enough  for  the  purpose  for  which  they  are  required,  and 
to  allow  the  alumina  to  entirely  penetrate  them  and  evaporate  itself  there- 
with ;  they  are  then  removed  and  allowed  to  dry  or  harden,  when  they  may 
be  dressed  and  polished  by  any  of  the  ordinary  and  well-known  processes 
for  polishing  ivory. 
Artificial  ivory  tablets,  capable  of  bearing  a  fine  polish,  may  also  be 
made  by  mixing  alumina  directly  with  gelatine  or  glue  ;  but  this  process 
is  not  so  satisfactory  as  the  process  hereinbefore  described,  since  the 
thickening  produced  by  the  admixture  of  alumina  with  the  gelatine  renders 
the  manufacture  of  the  sheets  both  difficult  and  expensive. 
Another  composition  of  artificial  ivory  which  is  employed  consists  of 
equal  portions  of  bone  or  ivory  dust,  used  either  separately  or  combined, 
and  albumen  or  gelatine,  the  whole  being  worked  into  paste,  and  after- 
wards rolled  out  into  sheets  by  suitable  rolling  or  flattening  mechanism. 
The  sheets  are  then  allowed  to  harden  by  exposure  to  the  atmosphere,  and 
are  cut  into  slabs  or  tablets  of  the  required  «ize.  But  it  is  preferred  to 
use  two  parts  of  fine  powdered  baryta,  and  one  part  of  albumen,  well 
worked  together,  and  rolled  out  into  slabs.  The  best  plan  hitherto  dis- 
covered for  working  the  materials  together,  is  that  commonly  used  in  the 
manufacture  of  Parian  marble;  this  composition  may  also  be  spread  upon 
paper,  if  desired.  These  slabs  or  tablets  are  then  carefully  scraped  to 
give  them  a  perfectly  even  surface.  They  are  then  washed  with  alcohol, 
to  remove  any  impurity  therefrom,  and  are  prtjDared  in  the  ordinary 
manner  to  receive  positive  pictures.  The  pictures  having  been  printed,  the 
entire  slab  or  tablet  may  be  immersed  for  a  few  minutes  in  a  weak  solu- 
tion of  nitro-sulphuric  acid  or  nitro-hydrochloric  acid,  for  the  purpose  of 
rendering  the  picture  more  clear  and  brilliant.  It  is  then  fixed  in  the 
usual  manner  with  hypo-sulphite  of  soda,  and  is  washed,  and  then  dried 
on  a  marble  or  other  slab,  or  under  pressure,  to  prevent  it  from  warping. — 
JjOndon  Pharm.  Joum.,  Oct.  1,  1857,  from  Lon.  Prac.  Mecli.  Jour. 
Preservation  of  Wood,  &c.  by  means  of  Coal  Tar  Oil. —  Creosote. — Dr . 
Vohl  recommends  mixing  the  creosote  with  caustic  soda  until  it  becomes 
miscible  with  water  in  any  proportion,  and  applying  this  liquid  to  the 
wood.  It  is  very  rapidly  absorbed,  and  when  the  wood  is  saturated  the 
creosote  is  fixed  by  immersing  the  wood  in  a  dilute  solution  of  sulphate  of 
iron.  The  sulphuric  acid  combines  with  the  soda,  the  creosote  with  the 
woody  fibre,  and  the  protoxide  of  iron  that  is  precipitated  is  subsequently 
converted  into  peroxide  by  the  atmospheric  oxygen  absorbed  by  the  wood, 
while  the  sulphate  of  soda  washes  out  when  the  wood  is  buried  in  the 
ground.  Wood  prepared  in  this  way  has  stood  for  eight  years,  exposed  to 
varying  influences  of  climate,  without  showing  any  indications  of  decay. 
Dr.  Vohl  states  that  the  coal  tar  oil,  commonly  called  creosote,  consists  for 
