METALS IN THK ATM* >SI'HKRK. 248 



liis ()l)S('r\iit ions on the mclcoi-itc of ()ri2ii(Ml. sonic of llicsc pai'liclcs 

 come fi-oni the explosion of luctcorilcs or from their simple disiinioii 

 when friiihh' and i)i'one to disintearation. Assnminf; that their mean 

 diameter is a constant of O.OI mm., whicli, in fact, is a rather too lib- 

 eral estimate, it woidd take -J. .'>()() of them to co\-er a s(|iiare millimeter 

 and ^r)().()(H) to e(|nal in hnlk a cubic millimetei". Therefore it is easily 

 ap})recialed that the deposit of iron on the earth's sui'face from this 

 source even in a <"onsideral)le inter\al of time Avill not be great. 



lint, admittinii' that some of the iron dust is of extraterrostrinl 

 ci'i^in. it is e(|u:dly true that a large ])art of it is swe])t from the 

 eartli"-- surface by the wind or-caiM'ied up in the smoke of the foun- 

 dries. In tlu' neighborhood of these works may be collected globules 

 of magnetic ii'on oxide, which, rising as s})arks. took the globular 

 form in cooling. It is easily shown that bits of iron at high temper- 

 atures become si)herical. and that a mass of it combining at red heat 

 with oxyg(Mi will divide into microscopic globular fragments. AVhen 

 very tine iron tilings are made incandescent by i)assing through a 

 hydrogen thuue. they burn brilliantly. Tissandier has discovered, 

 by collecting the products of this combustion on a porcelain plate 

 and examining them w itli a microscope, that they are in the fonn of 

 splun-ical globules, vase-necked spheres, irregular surfaced, or fibrous 

 fragments incompk^tely fused. Powder obtained by striking a ])iece 

 of ii-on with a flint is made up of the same kind of globules. An iron 

 wire burning in oxygen will form globules of magnetic oxide visible 

 to the naked eye and at the same time others nnich smaller, which 

 may be collected in water at the bottom of the flask. These are visi- 

 bl(> only under great magnification, since their diameter rarely exceeds 

 0.01 of a millimeter. 



The coml)ustion of coal in factories furnishes the air an abundance 

 of iron oxide from the decomposition of ferruginous pyrites contained 

 m the coal. I>ut all these particles obtained in the various w^ays men- 

 tioned, whether fi'om the combustion of iron, coal, or other substances, 

 are easily distinguishable from those of cosmic origin by the fact that 

 thev never contain nickel in anv form. 



Tilled land and salt water, whence gusts of wind snatch up parti- 

 cles the more minute of which measure scarcely 0.001 nun., often con- 

 tains nuignetic dusts in comparative abundance. A magnet passed 

 over their surface attracts tiny grains of magnetic iron oxide. This 

 is entirely independent of the particles due to the continual destruc- 

 tion of enormous (piantities of iron in manufacturing. The dust 

 from j)ulverized magnetic iron ore and other ferruginous minerals, 

 or the dust formed by oxidation in open air or from fresh or salt 

 water, never occurs in the mannnilated. fibrous, or spherical forms. 

 There are amorphic gi'ayish i)owders which do not resemble those 

 of planetary origin and are very dill'erent from those produced .it 



