METALS IX TlIK ATMOSI'HKKK. 245 



l)()lo, pyroxene, variously associate^l. All of (lie iiictcoric iiiiiicials 

 can be foiiiul in the dnsts of (he atniospheiv. 



Meteors nniy fall in a direction opposite^ to that of the earth's move- 

 ment, in which case their relative speed, beinj^ the sum of the two 

 movements, will he ver}^ great. })erhaps TO km. a second. The resist- 

 ance of the air to a flight of such s])eed produces enough heat to com- 

 pletely burn and volatilize the matter. 11", on the contrary, the move- 

 ment of the falling mass is in the same direction as that of the earth, 

 its relative velocity — the difference between the two absolute speeds — 

 is scarcely K) km. a second. In this case the heat developed is suffi- 

 cient only to fuse the mass and vitrify its surface, then perhaps to 

 break it in such a way as to form a meteorite or aerolite. It is not 

 sutKcient to make a shooting star with its great train of fiery particles. 

 Consequently, in coming into the earth's atmosphere, aerolites, whether 

 big oi- little, encounter a friction that generates heat and incandes- 

 cence, conse(|uently combustion, fusion, volatilization; condensation 

 of the volatilized particles follows, and the dissemination of these con- 

 densed particles. Thus it is easy to understand how meteorites bring 

 into the atmosphere \'arious metals, free or in combination, and why 

 metalliferous minerals, in cor])USc]es so minute that it is impossible 

 to separate and identify them, may be found in the air entirely inde- 

 ])endent of the pulverized minerals raised from the earth's surface. 



Matter .soli'h/c in tratcf. — As already mentioned, the " cinders " of 

 atmospheric sediments, when treated with water, always yield a cer- 

 tain amount of soluble salts. These are chlorates, alkaline sulj^hates, 

 or calcium sulphate, and nitrates, j^articularly that of annnonium. 



Aiiiinoniiiiii nitrate. — A drop of rain allowed to eva])()rate spon- 

 taneously on a bit of glass leaves crystals of various shapes on its 

 borders as the corpuscles are drawn toward the center. A star 

 with six points is well marked when the crystallization has taken 

 place slowly: more rarely the crystals assume the phnnule form. 

 Annnonium nitrate frequently forms remarkable groups of crystals 

 in the shape of crosses and swords, like those obtained by evaporating 

 a drop of snow water. In no other mannei- can similar crystals be 

 obtained, neither by varying the solution of the salts noi- the method 

 of evaporation. It will form only in regular crystals ramifying 

 uniforndy from a common stem or else in isolated prisms. Tissan- 

 dier attributes this peculiar crystallization in meteoric water to some 

 organic matter dissolved in the rain or snow. The evaporation of 

 this water leaves in the bottom of the vessel a hard, fragile residue 

 somewhat similar in appearance to coagulated albumen. Crystals 

 of ammonium nitrate are easily recognized by their solubility in 

 alcohol and by the fact that heat decomposes them without residue. 

 Their presence in the air can not l)e verified, since, as everyone knows, 

 nitric acid and annnonia unite readily to form ammonium nitrate. 



