558     DR.  HOOKER'S  ADDRESS  TO  THE  BRITISH  ASSOCIATION. 
vents  of  the  collodion,  and  so  become  damp.  This  suggested  to 
us  a  possible  use  for  rectifying  small  quantities  of  alcohol,  or  re- 
moving water  from  collodion  in  which  the  use  of  imperfectly- 
rectified  solvents  has  caused  a  tendency  to  give  crapy  films. 
Place  a  little  pure  gelatin  in  the  spirit  to  be  rectified.  There 
is  no  danger  of  any  portion  of  it  dissolving,  but  it  will  absorb 
the  water  and  gradually  swell ;  it  may  then  be  removed,  carry- 
ing the  water  with  it.  This  will  be  found  more  convenient  than 
the  plan  sometimes  recommended  of  agitating  with  carbonate  of 
potash,  and  after  subsidence  decanting. — Okem.  News,  Sept.  11, 
1868,  from  Photographic  News. 
HARD  AND  UNYIELDING  CEMENTS. 
By  M.  Schwartze. 
To  four  or  five  parts  of  clay,  thoroughly  dried  and  pulverized, 
add  two  parts  of  fine  iron  filings  free  from  oxide,  one  part  of 
peroxide  of  manganese,  one-half  of  sea  salt,  and  one-half  of 
borax.  Mingle  thoroughly  and  render  as  fine  as  possible ;  then 
reduce  to  a  thick  paste  with  the  necessary  quantity  of  water, 
mixing  thoroughly  well.  It  must  be  used  immediately.  After 
application  it  should  be  exposed  to  warmth,  gradually  increasing 
almost  to  white  heat.  This  cement  is  very  hard,  and  presents 
complete  resistance  alike  to  red  heat  and  boiling  water. 
Another  Cement. — To  equal  parts  of  sifted  peroxide  of  man- 
ganese and  well  pulverized  zinc  white,  add  a  sufficient  quantity 
of  commercial  soluble  glass  to  form  a  thin  paste.  This  mixture, 
when  used  immediately,  forms  a  cement  quite  equal  in  hardness 
and  resistance  to  that  obtained  by  the  first  method. —  Chemical 
News,  Sept.  18,  1868,  from  {Blatter  fiir  Gewerbe). 
EXTRACT  FROM  DR.  HOOKER'S  ADDRESS  TO  THE  BRIT- 
ISH ASSOCIATION  AT  NORWICH,  1868. 
In  this  most  unreliable  of  sciences — Fossil  Botany — we  do  but 
grope  in  the  dark  ;  of  the  thousands  of  objects  we  stumble 
against,  we  here  and  there  recognize  a  likeness  to  what  we  have 
elsewhere  known,  and  rely  on  external  similitude  for  a  helping 
hand  to  its  affinities ;  of  the  great  majority  of  specimens  we 
