EARTHS. 273 



(e.) Effects the most delicate test fluids ; if mixed with them in 

 substance* e. g. cabbage infusion, violet tincture, and that of tur- 

 meric ; but it is not sufficiently soluble in water, to impart the same 

 power to that fluid. 



(/.) Does not slack ivith water. 



(g.) Nearly insoluble in that fluid, which takes up about 5 T Vj at 

 60, and at 212 ^i^.f 



(A.) Absorbs water, so that 100 becomes, in weight, 118; heat 

 drives the water off, and the magnesia contracts again. It forms a 

 hydrate with water, but it unites with this fluid without any sensible 

 heat, and it is easily driven off at ignition. 



(i.) Precipitated from acids in the state of hydrate containing 

 probably one third water. 



(/.) This hydrate, dried by a very gentle heat, is transparent: it is 

 supposed to contain 1 equivalent of magnesia 20, and 1 of water, 9=29. 



(k.) Native hydrate, of Hoboken, New Jersey, contains about 30 

 per cent, of water. 



(I.) Alkalies do not combine with magnesia ; alkaline earths unite 

 with it by heat. 



(m.) Of very difficult fusion; first melted by Dr. Hare's blow* 

 pipe, in the laboratory of Yale College. J 



(ra.) Those minerals in which it is a large ingredient, are very 

 infusible ; hence soapstone is used in furnaces. 



(0.) With lirne, in excess, it melts in furnaces ; for the lime, al- 

 though itself infusible, acts as a flux. 



4. POLARITY. Magnesia goes to the negative pole, and is there- 

 fore electro- positive. 



5. COMBINING WEIGHT. Theory estimates it at 20 ; of which 12 is 

 assigned to magnesium and 8 to oxygen, being 1 proportion of each. 



6. CHARACTERISTICS. Its sulphate is very soluble, while those 

 of lime, baryta and strontia, are very insoluble : its nitrate and mu- 

 riate are very deliquescent,^ and soluble in alcohol : the bi-carbonates 

 of potassa and soda do not precipitate it, on account of the carbonic 

 acid. || Oxalate of ammonia, which readily precipitates lime, does not 

 precipitate magnesia, if the solution is moderately diluted. Turner. 



7. USES. Magnesia is a very useful article of the materia medica; 

 it is used as an antacid and cathartic. It seems however to be nearly 

 inoperative, unless there is acid in the stomach, or unless acid is 

 taken after it: all the salts of magnesia are bitter and cathartic. 



* Probably this effect is, in some cases, owing to the fact, that the alkali used in 

 decomposing the raagnesian salt has not been perfectly removed by washing. 

 t Fyfe, quoted by Henry. 

 t Con. Acad. Trans. Am. Jour. Vol. II, p. 290. 

 The nitrate of lime is deliquescent. 

 The same is true, in a good degree, of liinc. 



35 



