THE WORK OF DUST. 649 



Why is the sky in Italy and the tropics of a so much deeper 

 blue than that of western Europe ? Is the dust there finer ? It is 

 really so ; not that a finer quality of dust is produced there, but 

 because in the moist climate of the North Sea countries the dust 

 can not float long in the air without being charged with water 

 and made coarser, while in warmer countries water exists in the 

 air as vapor, and does not become condensed as a liquid on the 

 dust. Only when it is carried by the air currents into the higher 

 strata and is cooled there, does it thicken into clouds ? With 

 this we come to the most important function of dust in our at- 

 mosphere the part which it has in the function of rain by reason 

 of vapors condensing upon it. It can be affirmed with certainty 

 that all the water which the sun causes to evaporate on the sur- 

 face of the sea and on the land is condensed again on dust, and 

 that no raindrop falls unless it t had a particle of dust as its 

 primary nucleus. 



When we speak of "vapor" we always mean water in the 

 gaseous condition, transparent and invisible, like all other gases 

 but cloudy steam, such as is seen escaping from the boiler of a 

 locomotive. The latter, like the clouds and fogs, is liquid water 

 split up into innumerable fine drops. If the walls of a steam- 

 boiler were of glass, we should be able to see clearly through the 

 part of it occupied by steam. Then we have water in the gaseous 

 form. But when the steam escapes from it into much colder air, 

 it is condensed into liquid drops. The process is precisely the 

 same when the vapor which the sun has drawn up in the lower 

 warm strata of the atmosphere is cooled on rising, and forms 

 clouds. It is usually said that the upper atmospheric strata are 

 colder than the lower, because they permit a perfect passage of 

 the solar rays through them, and are therefore not warmed, while 

 the rays, on the other hand, warm the surface of the earth, and 

 that warms the air. This is true, but it does not explain why the 

 upper strata of the air do not become warmed in the course of 

 time. The supposition of a cooling of these strata by space does 

 not afford a sufficient cause, for a body which, like the air, stores 

 up little heat, likewise by a fixed law sends little out. Were the 

 atmosphere perfectly still it would, in fact, be warmed all through 

 from the earth's surface. But it is in constant motion, and the 

 heat is consequently very unevenly distributed through it. When 

 a column of air rises into the heights from the earth's surface, it 

 expands greatly, for the pressure to which it is subjected is much 

 less in the higher regions than below ; and whenever a gas ex- 

 pands it becomes colder. A quantity of heat is withdrawn from 

 it. corresponding with the force which it spends in expanding in 

 pushing itself into the surrounding region. Ascending air, there- 

 fore, becomes cooler, descending air warmer. Thus the fact is ex- 



