VARIETIES. 
185 
soluble  glass  or  freshly  precipitated  silica  renders  it  very  durable  and  at 
the  same  time  incombustible. — London  Chem.  News,  Dec.  29,  1865,  from  Wo- 
chenschr.  des  Nied.  B.  Oest.  Gew.-Vereins,  1865,  p.  502. 
Dentists'  Hard  Rubber. — A  paper  on  Hard  Rubber,  by  Dr,  Wildman , 
was  presented  and  read  by  Dr.  Buckingham,  before  the  American  Dental 
Association,  giving  the  results  of  some  of  his  experiments,  analytical  and 
synthetical.  Finding  the  manufacturers  of  the  rubber  compounds  for 
dental  purposes  extremely  reticent  as  to  its  composition,  and  but  little 
more  knowledge  to  be  gained  from  the  specifications  of  the  patents,  the 
Doctor  instituted  a  series  of  experiments,  to  obtain  a  better  knowledge  of 
the  value  of  the  different  compounds  vended  for  the  use  of  dentists.  A 
number  of  experiments  are  detailed  to  test  the  amount  of  earthy  matter  in 
different  rubbers  found  in  the  market ;  of  these,  the  American  Hard 
Rubber  Company's  left  five  and  four  per  cent,  respectively  of  ash,  one  of 
the  doctor's  make  three  per  cent.,  while  the  English  rubbers  and  others 
of  home  manufacture  left  a  residuum  of  from  forty  to  sixty  per  cent,  when 
brought  to  a  white  heat.  Proceeding  with  the  process  of  manufacturing 
different  samples,  we  learn  that,  in  making  rubber  compounds,  the 
caoutchouc  may  be  mixed  with  sulphur  and  the  coloring  matter,  by  being 
passed  repeatedly  between  steam-heated  rollers,  or,  for  experimental  pur- 
poses, the  caoutchouc  may  be  first  reduced  to  a  pulpy  state  by  some  one 
of  its  solvents,  and  the  sulphur  and  other  substances — being  previously 
ground  very  fine — thoroughly  incorporated  with  it.  The  solvents  used 
are  naphtha,  benzine,  or  oil  of  turpentine,  which  are  afterwards  evapo- 
rated by  exposure  of  the  mixture  to  the  air  on  plates  of  glass.  Several 
different  compounds,  of  various  colors,  are  described  in  their  composition. 
Caoutchouc  being  the  cement  which  binds  all  the  materials  together,  a 
glance  at  the  accompanying  table  will  show  the  great  inferiority  of  the 
English  pink  to  either  the  brown  or  red  rubber  for  dental  purposes. 
When  the  composition  contains  but  a  small  proportion  of  this  cement,  its 
weakness  of  texture  would  render  it  liable  to  produce  injurious  effects  by 
its  susceptibility  to  abrasion  in  the  mouth.  The  following  table  gives 
very  nearly  the  percentage  of  caoutchouc  contained  in  the  doctor's 
formulae,  and  also  that  of  the  English  pink  rubber  : 
Caoutchouc. 
Suiph. 
Verm. 
Sill.  Cad. 
Ox.  Zinc.    Parts  in 
Brown 
.      .  66* 
—  100 
Red 
.  44 
22 
300 
—  99 
Yellow 
.  44 
22 
33-0 
  99 
Pink 
•  42f 
21^ 
9  0 
27  100 
Buff 
.  35-4 
17-7x 
7-3 
4-4 
35-4x  100 
Drab 
.  44 
22 
33  99 
Light 
.  28'5x 
14'3x 
57-lx  100 
black. 
Black 
.  50 
25 
25  100 
Yellow  . 
.  40 
20 
40  100 
white  earthy  matter. 
English  pink  . 
.  24 
52 
12 
48  102 
— Dental  Cosmos. 
