VARIETIES. 
185 
Dental  Cosmos,  on  the  "Stone-knife,"  that  "the  serrations  become  clog- 
ged with  gold,"  reminds  me  to  say  that  the  serrations  of  our  instruments 
are  usually  made  on  a  wrong  principle.  They  are  made  with  a  fine  flat- 
sided  knife-shaped  file,  instead  of  with  a  file  with  curved  sides.  The  for- 
mer will  always  clog,  the  latter  never.  While  writing,  I  will  mention  a 
very  curious  effect  of  mercury  on  aluminium,  which  I  noticed  accidentally 
some  time  ago.  The  books  tell  us  that  they  will  not  unite,  but  I  under- 
took to  polish  a  piece  with  a  buckskin,  which  had  been  in  use  for  a  long 
time  for  squeezing  the  mercury  from  amalgam,  and  was  surprised,  while 
holding  the  aluminium  in  my  hand,  to  notice  a  great  heat  which  was  deve- 
loped from  it.  On  looking  at  it  closely,  I  was  still  more  astonished  to 
perceive  a  remarkable  efflorescence,  like  the  mould  on  old  cheese,  spring- 
ing up  and  growing  visibly  to  the  naked  eye.  I  pursued  the  experiment 
for  more  than  an  hour,  with  the  same  result.  Thinking  that  perhaps  it 
might  be  something  beside  the  mercury  which  induced  the  result,  I  took 
a  clean  piece  of  felt,  and  placed  some  mercury  upon  it,  wetting  it,  and 
breaking  up  the  globules  into  fine  particles  with  my  finger,  and  rubbing 
them  well  in.  Upon  rubbing  the  aluminium  with  this,  the  same  result 
was  again  produced.  Under  a  magnifying-glass  the  effect  is  very  fine. 
The  piece  of  aluminium  which  I  used  was  one  which  had  been  through 
the  vulcanizer,  under  Fowler's  process.  It  is  just  possible  that  the  sul- 
phur may  have  had  something  to  do  with  the  phenomenon. — Dental  Cos- 
mos, Feb.,  1867. 
A  Superior  Glue. — A  very  superior  glue  may  be  made  by  dissolving 
three  parts  of  India-rubber  in  thirty-four  parts  of  naptha.  Heat  and  agi- 
tation will  be  required  to  readily  effect  the  solution.  When  the  rubber  is 
completely  dissolved,  add  sixty-four  parts  of  finely  powdered  shellac,  which 
must  also  be  heated  in  the  mixture  until  all  is  dissolved.  This  mixture 
may  be  obtained  in  sheets  like  glue,  by  pouring  it,  when  hot,  upon  plates 
of  metal,  where  it  will  harden.  When  required  for  use  it  may  be  simply 
heated  in  a  pot  till  soft.  Two  pieces  of  wood  or  leather  joined  together 
with  this  glue  can  scarcely  be  sundered  without  a  fracture  or  tearing  of 
the  parts. — Drug.  Circ,  Jan.,  1867. 
Welding  Mixture. — Mr.  William  A.  Sweet,  of  Syracuse,  says  in  the  Sci- 
entific American:  "  I  send  you  a  recipe  for  using  on  cast  steel  in  welding, 
and  in  restoring  burnt  steel.  It  is  the  best  preparation  that  I  have  ever 
seen  or  used.  One  and  a  half  pounds  of  borax,  half  a  pound  of  sal  ammo- 
niac, quarter  of  a  pound  of  prussiate  of  potash,  one  ounce  of  rosin,  one  gill 
of  alcohol,  and  one  gill  of  water.  Pound  fine,  and  boil  in  an  iron  kettle 
slowly,  until  it  becomes  a  thick  paste.    Use  as  borax.--jr6i'cL 
Spongy  Platinum. — The  following  method  is  recommended  for  prepar- 
ing spongy  platinum  :  When  sal  ammoniac  is  added  to  a  solution  of  plati- 
