404 
ON  THE  NEW  METAL  THALLIUM. 
to  oxidation,  for  the  metal  precipitated  by  a  battery  from  an 
aqueous  solution,  or  fused  in  a  current  of  hydrogen,  is  white, 
with  a  bluish  gray  tinge,  which  resembles  aluminium. 
Thallium  is  very  soft,  and  very  malleable ;  it  can  be  easily 
scratched  by  the  nail,  and  cut  with  a  knife.  It  marks  paper, 
leaving  a  yellowish  streak.  Its  density  (11*9)  is  a  little  higher 
than  that  of  lead.  It  fuses  at  290°  C,  and  volatilizes  at  a  red 
heat.  Lastly,  thallium  has  a  great  tendency  to  crystallise,  for 
the  ingots  obtained  by  fusion  crackle  like  tin  when  they  are 
bent.  But  the  physical  property,  par  excellence,  of  thallium, — 
that  which,  according  to  the  beautiful  researches  of  MM.  Kirch- 
hoff  and  Bunsen,  characterises  the  metallic  element, — that  which 
led  to  its  discovery, — is  the  property  which  it  possesses  of  com- 
municating to  the  pale  gas-flame  a  green  color  of  great  richness, 
and  to  the  spectrum  of  this  flame  a  single  green  ray  as  distinct 
and  as  sharply  defined  as  the  yellow  ray  of  sodium,  or  the  red 
ray  of  lithium.  On  the  micrometric  scale  of  my  spectroscope, 
this  ray  occupies  the  division  120-5,  that  of  sodium  being  at  100. 
The  slightest  portion  of  thallium,  or  of  one  of  its  salts,  gives  the 
green  line  with  such  brilliancy  that  it  seems  white.  The  fifty- 
millionth  part  of  a  gramme  can,  according  to  my  calculations, 
be  recognised  in  a  compound. 
Thallium  tarnishes  rapidly  in  the  air,  becoming  covered  with 
a  thin  pellicle  of  oxide,  which  preserves  the  rest  of  the  metal 
from  alteration.  This  oxide  is  soluble,  is  decidedly  alkaline,  and 
has  a  taste  and  smell  similar  to  potash.  By  this  characteristic, 
as  well  as  by  its  optical  properties,  thallium  approaches  the 
alkaline  metals. 
Thallium  is  attacked  by  chlorine,  slowly  at  the  ordinary  tem- 
perature, rapidly  at  a  temperature  above  200°  C.  The  metal 
then  melts,  becomes  incandescent  under  the  action  of  the  gas, 
and  gives  rise  to  a  yellowish  liquid,  which  solidifies  on  cooling 
to  a  mass  of  a  little  paler  color. 
Iodine,  bromine,  sulphur,  and  phosphorus  can  also  combine 
with  thallium,  forming  iodides,  bromides,  sulphides,  and  phos- 
phides. 
Recently-prepared  thallium  preserves  its  metallic  lustre  in 
water.  It  does  not  appear  to  decompose  this  liquid  at  the  tem- 
perature of  ebullition,  but,  by  the  aid  of  an  acid,  it  separates  its 
elements,  disengaging  hydrogen. 
