520 



MAGNESIUM. 



the appearance of tarnished zinc. Faraday 

 states that the film of oxide, once formed over 

 a mass of the metal, protects it from further 

 action of the air. Unlike silver, magnesium is 

 not at all attacked by sulphuretted hydrogen 

 (sulphydric acid gas). The affinity of the metal 

 at elevated temperatures for oxygen is, however, 

 strong. When brought in contact even with 

 water- vapor, in a tube heated over an alcohol 

 lamp, it ignites, burning brilliantly, with disen- 

 gagement of hydrogen. This observation was 

 made recently by MM. Deville and Caron, who 

 also find that the metal, cold, decomposes water 

 in presence of the feeblest acids, even carbonic 

 acid. 



Thermic Equivalent of Magnesium. This 

 subject has been investigated by Dr. Thomas 

 "Woods, who states that his interest in the ques- 

 tion arose not alone from the fact of the great 

 amount of light yielded on combustion of mag- 

 nesium, but also in view of researches made 

 some years since by him on the amounts of heat 

 produced by the combination with oxygen of 

 several of the metals, and of the occurrence of 

 those amounts in multiple proportions (Philos. 

 Magas., Nov., 1852, and July, 1856). The gen- 

 eral method adopted by him, is that described 

 in the former of the two papers referred to 

 dissolving in dilute sulphuric acid, marking the 

 rise of temperature of the fluid, and correcting 

 the result by making the proper allowances for 

 the heat absorbed in the decomposition of wa- 

 ter, and that evolved by combination of the 

 magnesia with the acid. 



Combination of 4 grs. of zinc, or one equivalent, 

 oxygen=l (gr.), evolves heat sufficient to raise 

 the temperature of 1,000 grains of water by 9. 6 

 F. ; while combination of 1 grs. of magne- 

 sium, or one equivalent, 0=1, suffices fo raise 

 the temperature of 1,000 grs. of Avater 19.2 F. 

 The latter, the greatest quantity of heat afford- 

 ed by oxidation of an equivalent of any known 

 substance, is thus also just twice that developed 

 by oxidation of an equivalent quantity of zinc. 

 Of potassium and sodium, hitherto the greatest 

 heat-evolvers known, neither produces by oxi- 

 dation of one equivalent arise greater than 17.5 

 F. in 1,000 grs. of water. The quantity of 

 heat evolved by combination of magnesium 

 with chlorine is, however, greater than is af- 

 forded even with oxygen. Dissolving a metal 

 in chlorhydric acid, the acid is decomposed, 

 chlorine uniting with the metal. Adding to 

 the attendant rise of temperature the amount 

 of heat proper to the decomposition, we get the 

 heat actually generated by combination of the 

 chlorine with the metal. And it results that 

 one equivalent of magnesium, combining with 

 one of chlorine, evolves heat enough to raise 

 the temperature of 1,000 grains of water 25. 2 

 F. ; while one equivalent of zinc in like cir- 

 cumstances heats so much water 11.25; and 

 one equivalent of potassium, 22. 9. 



Qualities of the Magnesium Light. Of the 

 remarkable intensity of the light of burning 

 magnesium a wire only ^d (.012) of an inch 



in thickness affording for the time as ranch 

 light as seventy-four stearine candles it has 

 been offered in explanation that the great heat 

 due to oxidation of the metal is also developed 

 within a brief period and in a small mass of mat 

 ter, so that it is in consequence greatly concen- 

 trated, and gives rise to light-undulations of 

 corresponding rapidity (frequency) and force 

 (amplitude) ; the effect of such conditions 

 should be just such a light as the metal affords, 

 namely, one which is much more highly than 

 other artificial lights charged with the more re- 

 frangible rays blue, violet, and chemical, or 

 actinic, and which is also very intense. Com- 

 pared with other artificial lights, the lime- 

 light is highly bluish ; but it is stated that 

 when on a white screen the lime and magne- 

 sium lights are thrown side by side, the former 

 acquires by contrast a reddish hue. Dr. Woods' 

 investigations (see above) attest the preemi- 

 nent heat-yielding capacity of magnesium com- 

 binations. But the explanation of this great 

 heat is next to be sought ; and this too appears 

 now to be supplied in M. Schroder van der 

 Kolk's recent theory of a conversion of energy 

 accompanying chemical actions (See CHEMISTRY, 

 this volume) ; magnesia, chloride of magne- 

 sium, etc., must by this theory be regarded as 

 compounds the energy proper to which is very 

 greatly less than that representing the sum of 

 the energies of the elements entering into 

 them ; the lost energy of the act of combination 

 appearing as heat, which, under the usual con- 

 ditions of the case, suffers almost immediately 

 a further conversion in large degree into light 

 and actinism. And in fact, it would appear to 

 be through effect of the very conditions now 

 considered, that all the valuable qualities of the 

 magnesium light arise. 



Thus, containing so large a proportion of the 

 more refrangible rays, it is an achromatic or 

 white light, being in this respect closely similar 

 to sunlight. Hence, like the latter, and unlike 

 gaslight, which changes most colors to the eye, 

 and almost destroys some, it shows all colors 

 and hues perfectly, that is, without darkening 

 or other modification of them. Owing to the 

 intensity of the light, also, it possesses great 

 penetrating power, being visible to great dis- 

 tances ; while its richness in chemical rays ren 

 ders it admirably adapted to the purposes of 

 photography, in the night and in caverns, etc., 

 wherever in fact sunlight is not present, or its 

 chemical power is insufficient. M. Schrotter 

 has stated, indeed, that the spectrum of ultra- 

 violet rays obtained from the light of burning 

 magnesium is at least six times as long as its 

 luminous portion ; and M. Lallemand, agreeing 

 with M. Schrotter, finds that under the influ- 

 ence of the magnesium light,' as by sunlight, 

 hydrogen and chlorine gases in mixture com- 

 bine with explosion, an effect which does not 

 take place in darkness, nor under the influence 

 of the red or yellow rays. M. Chantard states 

 also (January, 1865), that the magnesium 

 light is well fitted to render phosphorescent 



