Oct,  r,  i.'396.] 
THE  TROPIC  VI, 
AGRICULTURIST. 
By  way  of  summary,  1 have  trhougho  iL  desirable  to 
indicate  ihe  general  characters  of  precious  stones  in 
a diagram,  which  exhibits  se  ine  of  (heir  rolationshipa 
and  also  some  of  their  differences  in  a graphic 
uianner. 
Opal,  which  is  a comparatively  light  mineral, 
has  a low  refractive  p'  wer  ; zircon  or  jargoon  is 
a heavy  mineral,  and  has  a high  ref:  active 
power.  Let  now  the  lefractive  power  of  any 
mineral  (as  measured  by  its  refractive  index 
for  yellow  light)  be  refiesented  by  a coires- 
ponding  length  set  off  fiom  left  to  right,  and  let  its 
density  (as  nie.asnrcd  by  its  specific  gravity)  be 
represented  by  a cerrer  pen  ding  Icrgtli  meatuied 
downwards.  Fixing  in  this  way  a point  correfponding 
to  opal,  and  another  representing  the  chaiactu-of 
zircon,  draw  a stiaight  line  fi om  the  one  to  the  other. 
It  will  then  be  found  that  the  points  which,  by  tluir 
position  on  the  diagram,  represent  the  specific  giavily 
and  refractive  index  of  the  vaiicus  minera's,  will  be 
very  neaily  upon  this  line;  that  is  to  say,  as  the  re- 
fractive index  of  precious  stones  increases,  so  also 
does  their  density,  and  the  two  incicase  together  in  a 
remarkably  regular  manner. 
It  appears  that  Ihcse  minerals  which,  by 
their  high  lefractive  power,  possess  the  greatest 
brilliancy,  possess  also  the  highest  specific 
gravity  "or  weightiness;  that  the  precious  stones 
are  therefore  all  heary  minerals.  There  is  also 
a rough  general  conespoiidence  between  these 
characters  and  the  hardness  of  the  stones;  the 
brilliant  heavy  minerals  are  also  generally  speaking 
hard. 
Tw’o  remarakable  exceptions  display  themselves. 
Spheue  lies  far  to  the  right  of  the  posiiicn  which  it 
should  occupy  according  to  its  specific  gravity ; it 
possesses  an  extraordinarily  high  refractive  index, 
and  is,  therefore,  an  extremely  brilliant  gem  stone. 
On  the  other  hand,  a glance  at  the  scale  of  hardness 
shows  that  it  is,  unfortunately,  one  of  the  possible  gem 
stones,  and  that  in  this  respect  it  is  not  very  well 
fitted  for  jewellery. 
Diamond  is  still  more  remarkable ; its  refractive 
index  places  it  at  the ' extreme  right  of  the 
diagram,  with  a refractive  power,  and  there- 
fore a brilliancy,  greater  than  that  of  any 
other  stone  ; at  the  same  time  its  hardness  exceeds 
that  of  any  mineral,  and  this  combination  of  quali- 
ties renders  it  the  chief  among  gem  stones,  un- 
equalled for  brilliancy  and  durability,  although  not  a 
heavy  mintral.  Moreover  in  dispersion,  and  therefore 
in  fire,  it  stands  alone.  Mirrerals  which  are  heavier 
than  zircODj  such  as  the  metallic  sulphides  and  iron- 
glance,  are  unsuitable  for  gem  stones  since  they  are 
nearly  opaqrre,  but  they  follow  the  some  law,  and 
possess  a refractive  power  still  greater  tbari  that  of 
zircon  or  even  diamond. 
There  is  one  other  stone  which  is  excep- 
tional but  iir  less  degree  and  in  the  other  direc- 
tion, namely,  topaz,  whose  refractive  index  is 
not  1’7,  as  it  should  be  by  its  position  on  the 
line  due  to  the  specific  gravitr , but  1’6‘2  ; the  point 
corresponding  to  topaz  must  therefore  be  placed  a 
short  distance  to  the  left  of  the  line.  It  is  curious 
that  these  three  exceptional  stones  lie  on  lire  same 
horizontal  line,  having  all  the  same  specific  gravity  o’5. 
In  mentioning  the  specific  gravity  I have  intro- 
duced a property  which  is  not  essential  to  win  esteem 
for  a precious  stone,  but  one  which  is  of  great  value 
in  its  identificatiorr. 
We  have  next  then  to  consider  those  properties  by 
which  precious  stones  may  in  practice  be  most  readily 
recognised.  The  table  shows  very  clearly  that  specific 
gravity  is  one  such  property.  The  meaning  of  spe- 
cific gravity  is  easily  explained.  A piece  of  tour- 
maline of  any  size  weighs  three  times  as  much 
as  an  equal  volume  of  pure  water  'at  4°  C.,  the 
specific  gravity  of  tourmaline  is  theiefore  said 
to  be  3;  a piece  of  alnrandine  garnet  of  any 
size  weighs  four  times  as  much  >s  an  equal  volume 
of  water  under  the  some  conditions,  and  the  specific 
gravity  of  garnet  is  therefore  4. 
Now  any  substance  immersed  in  water  loses  in 
weight  by  an  amount  exactly  equal  to  that  of  tho 
water  displaced,  Hence  to  ascertain  the  specific 
279 
gra--ity  it  is  only  ncce.ssary  to  suspend  the  stone  by 
a fine  tliieu'l  to  the  beam  of  a balance  and  weigh 
it  first  in  nir,  and  then  immeised  in  water.  The 
first  weighing  gives  the  weight  of  the  stone  itself, 
the  differtnee  between  the  first  weighing  and  the 
feccml  gives  the  weight  of  the  displaced  water; 
hence  the  specific  gia\ity  is  found  at  once  by  di- 
victiug  the  -weight  of  the  stone  by  this  difference. 
For  very  small  stones,  where  the  weights  concerned 
are  slight,  it  is  necessary  to  use  a refii'ed  chemical 
balance.  Hut  forcrdinaiy  stones  a well  made  West- 
plisl  balance  is  sufficient. 
The  Westplial  balance  is  consti noted  on  the  princi- 
ple cf  the  comn.on  steel  yard.  At  one  end  of  the 
beam  is  a counter  v^eight,  at  the  other  end  (he  stone 
is  iirfpti  ded;  the  beam  is  divided  into  ten  equal 
parts.  A weight  can  be  suspended  on  the  heani,  and 
its  action,  of  cenrse,  varies  with  its  po.Ttion  on  the 
beam;  at  the  tenth  division  from  the  centre  it  has  a 
value  ten  times  as  great  as  at  the  first  division. 
'J'hc  specific  gr  avity  is  then  found  as  follows  : — First 
counterpoise  the  counterweight.  Let  this  require  a 
weight,  A,  on  the  right  hand  side  of  the  beam.  Next 
liitd  the  weight  r.ccessaiy  to  restore  equilibrium 
■ft'hcn  the  stone  is  su.=pended  froitr  the  beam.  Let 
this  be  L.  Then  A — B is  the  weight  of  the  stone 
in  air.  Next,  raise  the  vessel  of  di.stilled  water  below 
the  stone  until  it  is  immersed.  If  G bo  the  weight  now 
r(qirii-ed  to  restore  cquiiibriunt,  G — B is  the  loss  of 
weight  in  water. 
A — B. 
and,  firallv,  the  specific  gravity  is 
G — B. 
This  process  is  known  as  “hydrostatic  weighing,” 
and  can  he  applied  to  any  stone,  except  such  as  are  very 
small.  Great  precautions  must  be  taken,  in  order  to 
determine  the  specific  gravity  with  accui  acy.  Especi- 
ally is  it  necessary  to  free  the  stone  from  all  adhering 
bubbles  of  air.  For  this  reason  the  process  of  hydros- 
tatic weighing  is  a somewhat  laborious  one. 
Now,  in  order  to  identify  a mineral,  it  ought 
to  be  urnecessary  to  determine  exactly  the 
specific  gravity,  provided  that  means  can  be 
devised  for  showing  that  its  specific  gravity  is 
the  same  as  that  of  some  known  substance. 
For  purposes  of  identification,  a comparative  method 
is  often  quite  as  efficacious,  and  much  more  easy 
than  acual  mea.‘-urcment.  This  may  now  be  done 
by  means  of  certain  heavy  liquids. 
Wood  floats  in  water  because  it  is  lighter  than 
water;  iron  sinks  because  it  is  heavier;  but  a sub- 
stance  which  possessed  exactly  the  specific  gravity 
of  water  would  neither  float  uc.r  sink,  but  would 
remain  suspended  in  the  water  like  a baloou  in 
mid  air.  Take,  then,  a liquid  which  is  heavr-— tho 
most  conrenient  is  methylene  iodide,  whose  specific 
gravity  is3:>— a fragment  c>f  zircon  will  sink  in  this 
and  a fragment  of  tourmaline  will  float,  but  a fragi 
merit  of  the  mineral  angite,  whose  spacific  gravity  la 
also  3'3.  will  exactly  remain  suspended,  ^ 
This  liquid,  then  enables  one  to  say  with  ccr. 
tainly  whether  a given  stone  has  a specific  -lavity 
greater  or  less  than  .S'3 ; in  the  one  case  it  will  sirlf 
in  the  other  it  will  float.  * 
But  methylene  iodide  furl  her  possesses  ihe  valuable 
property  of  mixing  cmiiy  with  benzene,  which  is  a 
very  light  liquid.  l..ciy  drop  of  benzene  added 
reduces  the  Bpeniu-  gravity  of  the  mixture,  which  can 
thus  easily  he  made  to  range  between  that  of  ebrv- 
solile  and  that  of  opal.  To  identify  any  one  of  the 
stones  which  lie  between  those  limits  on  the  diagram 
It  IS  only  iiccessary  to  drop  it  into  a test  tube  or 
small  vessel  containing  methylene  iodide— the  stone 
will  float — benzene  is  addecl  drop  by  drop  the 
mixture  being  kept  well  stirred  until  a point  is 
reached  at  which  the  stone  neither  sinks  nor 
floats.  Then  different  fragments  of  mineral 
possessing  specific  gravities  between  3’3  and  2’5  are 
taken  in  order  of  increasing  density  and  dropped  into 
tire  liquid  ; the  stone  under  examination  possess  a 
specific  gravity  between  that  of  the  last  which  floated, 
and  the  first  which  sinks,  and  the  limits  may,  if 
necessary,  be  further  narrowed  by  comparing  it  with 
other  mineral  fragments  of  known  density  intermediate 
between  those  two.  Gue  great  advantage  pf 
