ON THE DIFFERENT KINDS OF MAGNESIA, ETC. 279 
than hydrated magnesia, but especially because, having the 
property of combining with water, of being able to solidify 
ten times its own weight, it results that when it is intro- 
duced into the stomach in a large dose, from eight to six- 
teen grammes, for example, this oxide appropriates to itself 
the liquids contained in that organ, renders them solid, and 
dries the mucous membrane, which is found in some measure 
masticated by the hydrate of magnesia produced in this 
case. Hence the explanation of the thirst which always 
more or less accompanies the administration of a large dose 
of caustic magnesia; hence also the explanation- of that 
pinching, of that species of organic tenesmus which the pa- 
tient always suffer when they neglect to drink large draughts 
of water after swallowing it. 
This pathological state, occasioned by the employment 
of caustic magnesia, is so marked in certain persons that 
they are forced to suspend its use. 
Thus, for example, I have, from Dr. Emery, the calcined 
magnesia, which he has been obliged to renounce on ac- 
count of the indisposition caused by taken it, and to substi- 
tute for it the hydrocarbonate, which did not produce the 
same effect. Which difference of action that skilful practi- 
tioner explains by admitting in anhydrous oxide of magne- 
sium the presence of a certain quantity of caustic alkali, 
potassa, or soda in the free state, but which is very certain- 
ly attributable to the magnesia itself. 
From the foregoing, it indisputably results that practi- 
tioners will do well to renounce the use of recently calcined 
magnesia, and to substitute for it, not the hydrocarbonate 
of the shops, but, indeed, hydrated or slaked magnesia, that 
is to say, the hydroxide of magnesia obtained spontaneously 
in the air, like that which comes to us from England. Now, 
although a slight variation in the dose of a substance is in- 
nocent, as magnesia cannot bring about any bad result, it 
is, however, more rational to adopt the medical employ- 
ment of magnesia in invariable proportions ; that which ap- 
