The fog which we generally think of when we speak this word is the fog at or on the sea or other body of water--the one that makes the ships stand by and blow their fog horns. A fog of this kind is nothing more nor less than a cloud, come right down to earth and spread out a little more. People who have gone up into the air in balloons and other airships through the clouds, say that the clouds are only fogs, and that above them it is as clear as it is on a sunshiny day on the water when there is no fog. There is another kind of fog which settles down over the land, especially in the cities. It is a damp mist which combines with the smoke and other impurities in the air and forms a black and dirty cloud about everything. This occurs when the upper air prevents the smoke which rises from a city with all its people and fires in the furnaces from passing up and away. The upper air acts like a blanket and keeps the misty, smoky air down, until the wind comes along and blows it away. What Becomes of the Smoke? There are a number of things in smoke, and when we know what they are, we will find a natural answer to this question. First, there are, of course, the little unburned particles of fuel which get carried up the chimney by its drawing power. These naturally fall to the ground of their own weight, once they get beyond the drawing power of the chimney and out of the current of air so formed. Some of the gases are already quite burned out when they pass up the chimney. There is a lot of carbonic acid gas which, of course, mixes with the air and eventually becomes food for the plants. Then there are some gases which are not entirely burned, and the air burns them still more until they, too, become carbonic acid gas, or water which is also thrown off by a burning fire. Why Does an Apple Turn Brown When Cut? The reason is that when you cut an apple, the exposure to the air of the inside of the apple causes a chemical change to take place, due to the effect the oxygen in the air has on what is scientifically known as the enzymes in the apple, or what are commonly called the “ferments.” When the peel is unbroken it protects the inside of the apple against this action by the oxygen. The brown color happens to be due to the chemical action. The action is similar to the action of the air on wet or damp iron or steel, in which case we call it rust. Why Does a Piece of Wood Float in Water?