114 ON MR. PATTINSON's MAGNESIA ALBA. 
These numbers correspond almost exactly with those ob- 
tained in the analysis of a specimen of magnesia alba, pre- 
pared by myself in the usual manner, more than seven years 
ago — 100 parts gave, 
Magnesia, - - - - - 41.2 
Carbonic Acid, - 36.0 
Water, 22.8. 
100. 
Magnesia alba appears, in fact, to be a substance of very 
constant composition, although from the mode of its forma- 
tion, viz., the partial decomposition of the neutral carbonate 
first thrown down, by the water, in the old method of pre- 
paration, we might be led to think the reverse most likely 
to be true. 
It is well known to those engaged in Pharmacy, that 
there are met with in commerce two varieties of magnesia 
alba; the one exceedingly light and bulky, the other much 
denser. This difference arises solely from the state of me- 
chanical condition ; the two kinds having a chemical com- 
position sensibly the same, as will be seen by a comparative 
statement of their composition, determined as above. 
Light Magnesia Alba. Heavy Magnesia Alba. 
Magnesia, - 42.8 - - 41.2 
Carbonic Acid, 36.0 - - 36.4 
Water, - - 21.2 - - 22.4 
100. 100. 
A specimen of heavy magnesia alba prepared by myself, 
gave 
Magnesia, ----- 41.2 
Carbonic Acid, - 35.6 
Water, 23.2 
100. 
It is improper therefore to apply, as is sometimes done 3 
different names to the preparation. 
