ON THE HEAVY CALCINED MAGNESIA. 133 
pressed in a mould in order to give it compactness ; it is 
afterwards heated to whiteness for at least 6 or 8 hours." 
From the foregoing, it would appear that M. Durand would 
consider it necessary to employ a recently prepared and still 
hydrated carbonate of magnesia ; this alone renders his pro- 
cess very little practicable; and the white heat for 6 or 8 hours 
renders it very expensive. 
M. Colas's process is, on the contrary, so convenient, since 
it admits of a quantity of magnesia being calcined at one time 
and with less fuel, that it is to be regretted that his product 
cannot be recommended for medical use; ordinary calcined 
magnesia, notwithstanding the inconvenience of its lightness, 
should always be preferred to it, since it is much more soluble 
in the acids, and in equal weight is certainly more active. 
Before presenting M, Colas's product to the Societe de Phar~ 
made, I thought it my duty to repeat his experiments, which 
I found very accurate. I will here add some of mine, which, 
notwithstanding their little importance, may be placed after 
his, for they were made in consequence of them. 
In the first place, I do not think that M. Colas's process is 
exactly the same as that pursued by the English manufacturers ? 
for their heavy magnesia is so fine and soft to the touch that 
it might almost be taken for tale. This property is precisely 
the only one which M. Colas's magnesia does not possess inso 
high a degree. That which leads me to think thus is, that in 
these same manufactories, a heavy carbonate of magnesia is 
prepared, whose density is nearly equal to Henrys' magnesia, 
and that this calcined carbonate gives a product equally heavy, 
and very soft to the touch. 
The preparation of this carbonate is also a secret. 
I will conclude this note with a few words on the light cal- 
cined magnesia, which comes from London, and whose very 
moderate price is the cause of its being universally met with, 
and that many pharmaciens are wrong in not calcining their 
own magnesia. 
M. Mialhe (Journal de Pharmacie, June, 1844) finds that 
12* 
