closely packed, but light in form and small in size, taking the more flimsy appearance of fog. But if the dust-particles are fewer in proportion to the number of molecules of water-vapor, each particle soon gets weighted, becomes visible, and falls in mist or rain.
This can be shown by experiment. Let a jet of steam be passed into a glass receiver containing common air, and it will be soon filled with dense fog. Shut off the steam and allow the fog to settle. The air again becomes clear. Admit more steam, and the water-particles will seize hold of the dust-particles that previously escaped. Fog will be formed, but it will not be so dense. Again, shut off the steam, and allow the fog to settle and the air to clear. Then admit some steam, and very likely the condensed vapor will fall as rain. If the experiment be often enough repeated, rain instead of fog will be formed, because there are comparatively few solid particles on which the moisture can condense. When, then, dust is present in large quantities, the condensed vapor produces a fog; there are so many particles of dust to which the vapor can adhere that each can only get a very small share—so small, in fact, that the weight of the dust is scarcely affected by the addition of the vapor—and the fog formed remains for a time suspended in the air, too light to fall to the ground. But when the number of dust-particles is fewer, each particle can take hold of a greater space of the water-vapor, and mist particles or even rain-particles will be formed.
This principle that every fog-particle has embosomed in it an invisible dust-particle led Mr. Aitken to one of the most startling discoveries of our day—the enumeration of the dust-particles of the air. Thirty years ago M. Pasteur succeeded in counting the organic particles in the air; these are comparatively few, whereas the number of inorganic particles is legion. Dr. Koch, Dr. Percy Frankland, and others have devoted considerable attention to the enumeration of the micro-organisms in the air, and Mr. A. Wynter Blyth, the public analyst in London, has done good service in counting the micro-organisms in the different kinds of water in the vicinity. Marvelous as are the results, still the process was comparatively easy. By generating the colonies in a prepared gelatin, the number of microbes can be easily ascertained.
But to attempt to count the inorganic dust seemed almost equal in audacity to the scaling of the heavens. The numbering of the dust of the air, like the numbering of the hairs of the head, was considered as one of the prerogatives of the Deity. Yet Mr. Aitken has counted the "gay motes that people the sunbeams." Though he could not enlarge the particles by a nutritive process, as in the case of the organic particles, he has been able to enlarge them by transferring them into fog-particles, so as to
