132 ON THE HEAVY CALCINED MAGNESIA. 
least five times higher than that of the ordinary calcined 
magnesia. Except Messrs. Henry, Howard, and two or 
three other manufacturers, there are, probably, very few 
persons in England who possess the secret of this prepara- 
tion, and 1 think that M. Colas has rendered a true service 
to science by immediately making known the process which 
he has just discovered, and which consists in the following 
manipulation : — 
A very firm paste is made by moistening powdered car- 
bonate of magnesia. This paste should be well beaten, in 
order to employ as little water as possible ; then it is dried 
in an oven, and it is calcined after having been pressed 
down in the crucible. It also contracts by calcination. 
The crucible may also be filled with undried paste, and 
the whole put in the stove, in order to calcine it only after 
desiccation. 
M. Colas has proved that it requires less time and heat to 
calcine magnesia, thus prepared, than for the calcination of 
ordinary magnesia. He has also proved that his heavy mag- 
nesia does not become hydrated, even after having been left 
for 24 hours in cold water. Those of Henry and Howard do 
not become more hydrated ; it is likewise less soluble in the 
acids, and is much less apt to condense carbonic acid when ex- 
posed to the action of the air. 
In 1811, M. Planche (Bulletin de Pharmacie, vol. iii., p. 
511,) observed, that calcined magnesia might be obtained more 
or less dense, according as it is more or less strongly pressed 
before calcination ; but, in order to obtain it very heavy, he 
recommends it to be kept maintained at a white heat for 6 or 
8 hours. 
More recently M. Durand published a process more similar 
to that of M. Colas. 
This chemist expresses himself as follows — " Carbonate of 
magnesia is prepared with very pure sulphate of magnesia 
and carbonate of soda; before it is perfectly dry, it is strongly 
