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 atmosphere—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 surface of the sea and on the land is condensed again on dust, and that no raindrop falls unless it 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 expands 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, therefore, becomes cooler, descending air warmer. Thus the fact is ex-
