Page:Popular Science Monthly Volume 17.djvu/361

Rh the incandescent dust of aërolites. Baron Reichenbach collected on the summit of Lahisberg a black ferruginous dust containing traces of nickel and cobalt, which incontestably indicated its cosmic origin. Mr. Nordenskjöld collected a similar dust from off the snow of the polar regions.

For several years I have paid attention to the study of atmospheric dust, and believe that I have proved that more or less considerable quantities of dust derived from cosmic bodies are constantly present in the air. The greater part of my experiments have been performed in the meteorological observatory of Sainte-Marie-du-Mont, Manche, where M. Hervé-Mangon has placed the resources of his establishment and laboratory at my disposition. I have endeavored to collect large quantities of dust, so that I might carry on microscopic and chemical analyses with precision. For this purpose I used a surface of paper of two square metres, which I exposed horizontally to the air; I collected from it the dust which fell from the air, sweeping it up with a small brush. I called this apparatus a dust-table. The weight of the dust collected upon this surface at Sainte-Marie-du-Mont varied from two to nine milligrammes (.03 to .14 grains troy) in twenty-four hours. The particles shown in Fig. 1 represent minute grains of magnetic oxide of iron, which were drawn out from the dust by the magnet. They are greatly magnified, their real diameter being only of a millimetre  of an inch).

If we evaporate considerable volumes of rain-water, we shall obtain a sediment which represents the air-dust; if, then, we draw a magnet through this sediment, we shall nearly always find little globules of magnetic oxide of iron in it. Fig. 2 represents some of these globules, which I extracted from one hundred quarts of rain-water at Sainte-Marie-du-Mont.

We can distinguish several of these grains which have the form of spherules, or granules that have undergone fusion. The sediment from rain-water collected in Paris at the School of Bridges and Highways at the Trocadéro and the dust collected in one of the towers of Notre Dame gave identical results (Fig. 3).



These facts prove conclusively that the air holds in suspension minute microscopic particles of oxide of iron, some of which assume the form of well-defined spherules.

Similar spherules may be found everywhere in the dust of the air and in rain-water and snow-water. I have found them in the