PREPARATION  OF  HEAVY  CALCINED  MAGNESIA. 
199 
of  the  question,  but  leave  it  with  the  hope  that  abler  hands  than 
mine  will  give  an  answer. 
The  fused  subchloride,  when  freshly  broken,  presents  a  beauti- 
ful striated  fracture.  A  dilute  solution  of  the  chloride  allowed  to 
evaporate  spontaneously  on  a  plate  of  glass,  deposited  crystals, 
which,  in  their  mode  of  formation  and  general  appearance,  re- 
sembled those  of  nitrate  of  ammonia.  They  were,  however,  very 
deliquescent,  and  could  not  therefore  be  long  preserved. 
A  specimen  (No.  10)  of  hydrate  of  magnesia  in  mass,  contami- 
nated with  iron,  is  presented  with  the  other  specimens.  The  re- 
sults of  the  analyses  previously  referred  to,  of  different  ponderous 
magnesias  of  commerce,  are  here  presented  in  tabular  form.  They 
have  been  made  with  care  and  attention  to  accuracy. 
Henry's 
Hugband's, 
Ellis', 
T.  H.  Barr's  Specimen, 
Sp.  gr.  3.404. 
Sp.  gr.  3.326. 
Sp.  gr.  3.386. 
No.  7.    Sp.  gr.  3.442. 
Magnesia,  .    .  . 
94.40 
84.306 
94.04 
95.016 
Water,  .... 
.50 
11.400 
.80 
1.300 
Silica,  .... 
.80 
Trace 
Trace 
Trace 
Iron,   
1.20 
.700 
.80 
.400 
Lime,  .... 
.11 
1.008 
1.81 
-404 
Alumina,    .    .  . 
1.80 
.500 
.60 
1.200 
Sulph.  Magnes.  . 
•.30 
.300 
.50 
•800 
Sulph.  Soda,    .  . 
1.60 
1.100 
.70 
.400 
Chlorine,    .    .  . 
Trace. 
Trace. 
100.71 
99  314 
99.25 
99.520 
There  is  one  point  in  particular  in  the  analyses  to  which  I  would 
like  to  call  attention,  and  that  is  the  large  proportion  of  silica 
in  Henry's  magnesia  compared  with  that  contained  in  the  others. 
The  greater  part  of  the  silica  in  the  article  referred  to,  was  left 
behind  as  a  gelatinous  precipitate  with  a  little  anhydrous  silica. 
A  precipitate  of  this  kind  is  produced  only  when  a  silicate  of  some 
base  is  decomposed.  It  is  supposed,  and  I  think  with  reason,  that 
Henry's  magnesia  is  calcined  at  a  very  high  temperature,  and  as 
magnesia  will  unite  with  silicic  acid  (which  is  contained  in,  almost 
if  not  all,  crucibles)  at  a  high  heat,  I  think  that  this  will  account 
for  the  presence  of  that  impurity  in  such  a  large  proportion.  The 
amount  of  silica  contained  in  the  others  is  a  mere  trace. 
The  amount  of  water  found  by  me  in  Henry's  and  Ellis's  mag- 
nesias approximates  closely  with  that  found  by  Prof.  Procter, 
(see  Am.  Journal  of  Pharmacy,  Vol.  22,  page  383,)  but  with 
Husband's  there  is  a  considerable  difference  in  the  results,  showing 
