586 PEINCIPLES OF CHEMISTRY 



is a white metal, like silver ; it is not soft like the alkali metals, but 

 is, on the contrary, hard like the majority of the ordinary metals. 

 This is natural from the fact that it melts at a rather high tempera- 

 ture namely, about 500 and boils at about 1000. It is malle- 

 able and ductile, like the generality of metals, so that it can be drawn 

 into wires and rolled into ribbon ; it is most frequently used for 

 lighting purposes in the latter form. Unlike the alkali metals, mag- 

 nesium does not decompose the atmospheric moisture at the ordinary 

 temperature, so that it is almost unacted on by air ; it is not even 

 acted on by water at the ordinary temperature, so that it may be 

 washed to free it from sodium chloride. Magnesium only decomposes 

 water with the evolution of hydrogen at the boiling point of water, 15 

 and especially at still higher temperatures ; but even this is accom- 

 plished with difficulty. This is explained by the fact that in decom- 

 posing water magnesium forms an insoluble hydroxide, MgH 2 O 2 , which 

 covers the metal and hinders the further action of the water. Magne- 

 sium easily displaces hydrogen from acids, forming magnesium salts. 

 . When ignited it burns, not only in oxygen but in air (and even in 

 carbonic anhydride), forming a white powder of magnesium oxide, or 

 magnesia ; in burning it emits a white and exceedingly brilliant light. 

 The strength of this light naturally depends on the fact that mag- 

 nesium (24 parts by weight) in burning evolves about 140 thousand 

 heat units, and that the product of combustion, MgO, is infusible by 

 heat ; and therefore the vapour of the burning magnesium will contain 

 an ignited powder of non-volatile and infusible magnesia, and will 

 consequently present all the conditions for the production of a brilliant 

 light. The light emitted by burning magnesium contains many rays 

 which act chemically, and are situated in the violet and ultra-violet 

 parts of the spectrum. For this reason burning magnesium may be 

 employed for taking photographic images. 16 



Owing to its great affinity for oxygen, magnesium reduces many 

 metals (zinc, iron, bismuth, antimony, cadmium, tin, lead, copper, silver, 

 and others) from solutions of their salts at the ordinary temperature, 17 



i:> Hydrogen peroxide (Welsing) dissolves magnesium. The reaction has not been 

 investigated. 



10 A special form of apparatus is used for burning magnesium. It is a clockwork 

 arrangement in which a cylinder rotates, round which a ribbon or wire of magnesium is 

 wound. The wire is subjected to a uniform unwinding and burning as the cylinder 

 rotates, and in this manner the combustion may continue uniform for a certain time. 

 The same is attained in special lamps, by causing a mixture of sand and finely-divided 

 magnesium to fall from a funnel-shaped reservoir on to the flame. In photography it is 

 best to blow finely-divided magnesium into a colourless (spirit or gas) flame. 



17 According to the observations of Maack, Comaille, Bb'ttger, and others. The re- 

 duction by heat mentioned further on was pointed out by Geuther, Phipson, Parkinson 

 and Gattermann. 



