276 ON DIFFERENT KINDS OF MAGNESIA, ETC. 
tains little or no carbonic acid. It always contains a very 
great quantity of water, varying in proportion from 12 to 
20 per cent., as I have ascertained by experiment. 
For some time I thought that I was the first to prove the 
presence of water in light English calcined magnesia, but I 
hasten to declare that this is not the case. M. Dubai], in 
his excellent Inaugural Thesis, has devoted an article to it, 
from which I will quote some passages which leave nothing 
to be desired : — 
"For some time decarbonated magnesia has come to 
us from London, it appears, which is sold at a lower price 
than it can be manufactured for here, whatever economy 
be used. 
" This magnesia is rather lighter than ordinary calcined 
magnesia ; it dissolves like it, and more easily, without ef- 
fervescence, in the acids. It gives no indication of any 
foreign base — it is therefore regarded as pure ; but, when 
calcined, it loses 20 per cent, of water." 
What is the true nature of this variety of decarbonated 
magnesia? I had presumed for an instant that this oxide 
of magnesium should constitute a hydrate in definite pro- 
portions ; the same idea occurred to M. Dubail, but I was 
very soon convinced of the contrary ; it is a mixture of hy- 
drate and non-hydrated oxide ; it is a hydroxide in variable 
proportions.* 
By what process has this decarbonated magnesia been 
obtained ? 
This calcined magnesia has certainly been prepared by 
the ordinary method, namely, by the calcination of the hy- 
drocarbonate \ but it has afterwards been exposed to the 
damp air for a more or less long time, or it has been pur- 
posely exposed, which is probable, or it has accidentally 
become exposed during the journey; however, by exposing 
recently calcined magnesia to humid air, I have produced, 
in two or three days, an hydroxide of magnesia in every 
* The true hydrate of magnesia contains 30 per cent, of water. 
