the late Mr. Henry. 211 
terwards it was reprinted, along with essays 
on other subjects, in a separate volume, which 
was dedicated by Mr. Henry to his friend Dr. 
Percival. 
The calcination of magnesia had, at that 
time, been practised only in connection with 
philosophical inquiries. Dr. Black, in an Es- 
say which is still perhaps not surpassed in 
chemical philosophy, as an example of in- 
ductiveinvestigation, had fully established the 
differences between magnesia in its common 
and in its calcined state ; but he does not ap- 
pear to have made trial of the pure earth as 
a medicine, though several inconveniences, 
from its use in the common form, had long 
before been pointed out by Hoffman.* On 
this subject Mr. Henry’s claims extend to the 
free disclosure of his improvements ; to the 
early and strenuous recommendation of the 
medicinal use of pure magnesia; and to the 
discovery of some of its chemical agencies. 
It is but justice to him to state that his re- 
commendation of its employment as a medi- 
cine was perfectly disinterested ; for it was 
not till his work was printed, and on the eve 
of issuing from the press, that the preparation 
of magnesia for sale was suggested to him by. 
* Hoffman. Oper. Tom. 4. p. 381. 
