134 ON THE HEAVY CALCINED MAGNESIA. 
" this kind of magnesia contains little or no carbonic acid." 
For my own part, I have never met with English calcined 
magnesia of commerce which did not contain carbonic acid ; 
on the contrary, a very appreciable quantity of it is always 
found, and it always effervesces with sulphuric acid diluted to 
one-eighth. With respect to the water which it contains, his 
observations and those of M. Dubail are too correct for me to 
add anything to them.' I will only say to M. Mialhe, that I 
do not think, with him, that ordinary calcined magnesia at- 
tracts carbonic acid only when it is hydrated ; I think that it 
attracts water and carbonic acid at the same time, and it was 
with the view of proving my assertion that I made the follow- 
ing experiment : — 
Light caustic magnesia* was left for a month over mercury, 
and in an atmosphere of perfectly dried carbonic acid. Not 
the slightest absorption took place. I afterwards placed in the 
top of the bell-receiver a piece of ice wrapt in blotting-paper, 
in order that it might traverse the magnesia without wetting 
it. The carbonic acid was there gradually absorbed in so short 
a space of time, that, in this case, the magnesia could not have 
been hydrated before absorbing the gas. I think that the 
latter explains the presence of carbonic acid in the light cal- 
cined magnesia which comes to us from England ; for, as M. 
Mialhe has wisely supposed, the English manufacturers are in 
the habit of exposing their calcined magnesia to the humid air, 
in order thus to obtain an increase of weight,varying between 
15 and 20 per cent. I have several times repeated this experi- 
ment, and, contrary to the opinion of M. Mialhe, I have never 
succeeded in obtaining an hydroxide, but always a mixture of 
hydroxide and carbonate. An hydroxide with 1 atom of water 
may be directly obtained, provided that the magnesia be very 
caustic, and especially very light ; if it possesses these two 
properties, it will be seen that, after having been wetted, by 
* I give this name to the magnesia which we prepare in our labora- 
tories to distinguish it from English magnesia. 
