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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 pre- 

 viously 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 re- 

 peated, rain instead of fog will be formed, because there are com- 

 paratively 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 sun- 

 beams." 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 



