596 PRINCIPLES OF CHEMISTRY 



evolves about 140 thousand heat units, and that the product of com* 

 bustion, MgO, is infusible by heat ; so that the vapour of the burning 

 magnesium contains an ignited powder of non-volatile and infusible 

 magnesia, and consequently presents all the conditions for the pro- 

 duction 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 producing 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 

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

 silica, alumina, boric anhydride. <fec. ; 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 



The affinity of magnesium for the halogens is much more feeble 

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

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

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

 is also seen in the fact that all its salts, especially in the presence 

 of water, are decomposable at a comparatively moderate tempera- 

 ture, the elements of the acid being evolved, and the magnesium 

 oxide, which is non- volatile and unchangeable by heat, being left. 

 This naturally refers to those acids which are themselves volatilised 

 by heat. Even magnesium sulphate is completely decomposed at 

 the temperature at which iron melts, oxide of magnesium remaining 

 behind. This decomposition of magnesium salts by heat proceeds 



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, and for 

 instantaneous photography to light a cartridge of a mixture of magnesium and chlorate 

 of potassium by means of a spark from a Ruhmkorff s coil (D Mendeleeff, 1889). 



17 According to the observations of Maack, Comaille, Bottger, and others*, The re- 

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

 and Gattermann. 



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

 (see Note 13), on its volatility, and on the fact that, in combining with a given quantity of 

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



19 Davy, on heating magnesia in chlorine, concluded that there was a complete sub- 

 stitution, because the volume of the oxygen was half the volume of the chlorine ; it is 

 probable, however, that owing to the formation of chlorine oxide (Chapter XI., Note 30) 

 th decomposition is not complete and is limited by a reverse reaction. 



