200 
PREPARATION  OF   HE AVY  CALCINED  MAGNESIA. 
that  different  specimens  differ  in  the  amount  of  water  which  they 
contain. 
The  true  hydrate  of  magnesia  contains  about  thirty  per  cent,  of 
water,  and,  as  I  have  shown  in  this  essay,  is  gritty,  and  not  a 
desirable  article,  from  which  it  would  seem  that  Husband's,  like 
the  ordinary  English  calcined,  is  only  partially  hydrated,  proba- 
bly by  the  gradual  absorption  of  water  after  leaving  the  fire.  This 
is  rendered  more  probable  from  the  known  fact  that  caustic  mag- 
nesia has  a  much  stronger  affinity  for  water  than  for  carbonic  acid. 
My  attempt  to  improve  the  physical  properties  of  a  specimen  by 
passing  steam  into  it  failed,  and  I  have  found,  moreover,  that 
Husband's  magnesia  is  softer  after  being  dehydrated  than  before, 
so  that  whatever  effect  water  may  have  upon  the  therapeutic  ac- 
tion of  this  medicine,  I  do  not  believe  it  is  calculated  to  improve 
its  softness  or  impalpability. 
The  subject  of  this  paper  has  occupied  as  much  time  and  atten- 
tion as  I  can  now  devote  to  it ;  but  I  am  convinced  that  the  whole 
question  is  still  worthy  of  careful  investigation,  with  a  view  to 
ascertain  not  only  the  cheapest  and  most  productive  process,  but 
also  which  of  the  products  obtained  are  most  worthy  the  esteem  of 
the  medical  practitioner. 
If,  in  the  course  of  these  experiments,  I  have  elicited  any  new 
facts  connected  with  this  often  mooted  subject,  of  interest  or  ad- 
vantage to  the  profession  of  Pharmacy,  my  labor  will  not  have 
been  in  vain. 
It  is  proper  for  me  to  state  in  this  place,  that  the  experiments 
detailed  in  this  paper  were  made  in  the  laboratory  of  ray  preceptor, 
Mr.  Edward  Parrish,  and  I  acknowledge  my  obligations  to  him  for 
the  opportunities  afforded  me  for  prosecuting  my  investigations. 
Note. — [We  have  examined  the  three  specimens  of  magnesia,  referred  to 
at  page  196,  as  regards  their  texture  and  appearance,  in  comparison  with 
Henry's  magnesia.  Specimen  No.  6  agrees  very  closely  with  the  best  heavy 
magnesias  of  commerce,  yet  it  lacks,  in  a  readily  appreciable  degree,  the 
velvety  softness  of  genuine  Henry's  or  Husband's  magnesia,  or  that  of  the 
Messrs.  Ellis,  As  a  first  result  it  is  very  successful,  and  speaks  well  for 
the  skill  of  Mr.  Barr. — Editor.] 
