THE WORK OF DUST. 647 



THE WORK OF DUST. 



By Dr. P. LENAED. 



WHEN a beam of sunliglit enters a darkened room through a 

 hole in the window shutter, it can be seen along its whole 

 course. The light is reflected to every side, and made to reach 

 the eye by the dust in the air of the room. "We do not see the 

 sunbeam itself, but the dust which is illuminated by it ; and indi- 

 vidual bodies can be perceived on a closer inspection floating in 

 the beam. The dust may be much more plainly observed in still 

 air, as it settles on objects. It is extremely slow in falling to the 

 ground, although it consists of matter which in larger masses falls 

 very speedily. This we can test by collecting dust and compress- 

 ing it into a ball. In this process of compression a very large 

 part of the exposed surface which the particles presented to the 

 air is caused to disappear; and it was by means of this great 

 extent of surface that the air bore enough upon the particles to 

 support them against falling. The finer the dust the more ex- 

 tended is its surface in proportion to its mass, and the more 

 it is delayed in falling through the air. It may seem useless 

 to speak of the part played by this dust in Nature; for what 

 noticeable effect can this insignificant stuff bring about ? We 

 have, however, as can be shown, no right to regard it as a little 

 thing. 



Dust has a very large share in nearly all the phenomena of 

 the earth's atmosphere. It is what makes the clear sky appear 

 blue ; and when we look up into the sky we see the dust in the 

 atmosphere illuminated by the sun. There is nothing else before 

 us that can permit the light to reach the eye. Light goes invis- 

 ible, straight through all gases, whatever their chemical compo- 

 sition. The dust catches it, reflects it in every direction, and so 

 causes the whole atmosphere to appear clear, in the same way 

 that it makes the sunbeam visible in the darkened room. With- 

 out dust there would be no blue firmament. The sky would be 

 as dark as or darker than we see it in the finest moonless nights. 

 The glowing disk of the sun would stand immediately upon this 

 dark background, and the same sharp contrast would prevail 

 upon the illuminated surface of the earth blinding light, where 

 the sun's rays fall, and deep black shadows where they do not. 

 Only the light of the moon and the stars, which would remain 

 visible in the daytime, would be able to temper this contrast in 

 a slight degree. The illumination of the earth's surface would 

 be like that we see with the telescope on the lunar landscapes ; 

 for the moon has no atmospheric envelope that can hold floating 

 dust. We then owe to dust the even moderately tempered day- 



