MAGNESIUM. 55 



Source. Made like magnesiae carbonas, but with 

 cold dilute solutions instead of hot. 



Characters. A very light white powder, proving 

 microscopically to be partly amorphous, partly pris- 

 matic crystals. Soluble in 2,493 parts of cold water, 

 or in 9,000 parts of hot water ; 3^- times the bulk of the 

 heavy carbonate. 



Dose. 10 to 60 gr. 



From Magnesia Carbonas Levis is made : 



Magnesias Levis. Light Magnesia. MgO. 



Soiu-ce. Made by heating Light Carbonate of 

 Magnesia jn a crucible to expel the carbonic acid. 



Characters. A bulky white powder, 3J times 

 the bulk of heavy magnesia. 



Dose. 10 to 60 gr. 



Magnesia Carbonis Levis is contained in Pulvis 

 Rhei Compositus (6 parts in 9). 



ACTION AND USES. 

 1. IMMEDIATE LOCAL ACTION AND USES. 



Externally, magnesia has no action, and is not used. 



Internally, magnesia is a valuable means of decomposing the 

 contents of the stomach and intestines under various circum- 

 stances. The base and carbonates form comparatively insoluble 

 or innocuous compounds with the mineral acids, oxalic acid, 

 mercuric, arsenical, and cupric salts ; in large quantities they 

 prevent the absorption of alkaloids by rendering the contents 

 of the stomach alkaline ; whilst the sulphate precipitates inso- 

 luble sulphates of lead and baryta. Magnesia or its salts may 

 therefore be employed as antidotes in cases of poisoning by 

 any of these substances, the oxide being preferred to the car- 

 bonate, so as to prevent the evolution of carbonic acid, and 

 care being taken to give it very freely. 



By a similar process of decomposition, magnesia neutralises 

 normal or excessive acidity in the stomach and bowels, and is 

 itself converted into the chloride, lactate, and bicarbonate, this 

 reaction removing irritant acid, and forming salts of magnesia, 

 which have a stimulant or purgative action on the intestine. 

 The carbonate is similarly decomposed, yielding carbonic acid, 

 which exerts its specific action on the stomach. Both sub- 

 stances are therefore employed as local alkaline remedies in 

 acidity of the stomach (heartburn, pyrosis, etc.), given with sal- 

 volatile, after meals, a further laxative effect on the intestine 

 being intended. 



