TIIK YALENTY AND SPECIFIC HEAT OF THE METALS 587 



and at a red heat finely-divided magnesium takes up the oxygen from 

 silica, alumina, boric anhydride, Arc. ; so that silicon and similar 

 elements may be obtained by directly heating a mixture of powdered 

 silica and magnesium in an infusible glass tube. 18 Magnesium acts on 

 fused potassium and sodium hydroxides, with an energetic evolution of 

 hydrogen. 



The affinity of magnesium for the halogens is much more feeble 

 than for oxygen, 19 as is already evident from the fact that a solution 

 of iodine reacts feebly on magnesium ; still magnesium burns in the 

 vapours of iodine, bromine, and chlorine. The character of magnesium 

 is also determined by the fact that all its salts, especially in the 

 presence of water, are decomposable at a comparatively moderate 

 temperature, the elements of the acid being evolved, and the magne- 

 sium oxide, which is non-volatile and unchangeable by heat, remaining 

 behind. This naturally refers to those acids which are themselves 

 volatilised by heat. Even magnesium sulphate is completely decom- 

 posed at the temperature at which iron melts, oxide of magnesium 

 remaining behind. This decomposition of magnesium salts by heat 

 proceeds much more easily than with calcium salts. For example, 

 magnesium carbonate is totally decomposed at 170, magnesium oxide 

 being left behind. This magnesia, or magnesium oxide, is met with 

 both in an anhydrous and hydrated state in nature (the anhydrous 

 magnesia as the mineral periclase, MgO, and the hydrated magnesia 

 as hrucite, MgH 2 O 2 ). Magnesia is a well-known medicine (calcined 

 magnesia magnesia usta). It is a white, extremely fine, and very 

 voluminous powder, of specific gravity 3*4 ; it is infusible by heat, and 

 only shrinks or sinters in an oxyhydrogen flame. After long contact 

 the anhydrous magnesia combines with water, although very slowly, 

 forming the hydroxide Mg(HO) 2 , which, however, parts with its 

 water with great ease when heated even below a red heat, and again 

 yields anhydrous magnesia. This hydroxide is obtained directly as 

 & gelatinous amorphous substance when a soluble alkali is mixed with 

 a solution of any magnesium salt, MgCl 2 + 2KHO = Mg(HO) 2 + 2KCl. 

 This decomposition proceeds to the end, and nearly all the magnesium 

 passes into the precipitate ; and this clearly shows the almost perfect 

 insolubility of magnesia in water. Water dissolves a scarcely per- 



18 This action of metallic magnesium in all probability depends, although only partially 

 (see Note 18), on its volatility, and on the fact that, in combining with a given quantity of 

 oxygen, it evolves more heat than aluminium, silicon, potassium, and other elements. 



19 Davy, on heating magnesia in chlorine, observed a complete substitution, because 

 the volume of oxygen was half the volume of chlorine ; it is probable, however, that owing 

 to the formation of chlorine oxide (Chapter XI. Note 80) the decomposition is not com- 

 plete and is limited by a reverse reaction if the mass of the oxygen have no effect. 



