An illustration may help to explain this. Suppose you scatter a spoonful of small shot over a carpet or a dark-coloured table-cloth; you would probably not be able to see them at a little distance. Now, gather them together in a heap, and you see them at once. The heap of shot in some ways resembles a drop of water, for in a drop of water the tiny particles (or molecules) of which it is composed are close together; but by heating water you cause them to fly asunder and scatter themselves in various directions. They are lost to sight, and moreover have no power of attracting each other or of acting in concert; each one then takes its own course, whereas in the drop of water they were in some wonderful way bound together by mutual attraction. They dance in groups; but the rude force of heat will scatter these little dancing groups, and break them up into that state which we call a state of vapour. The forces of heat and cohesion are directly at variance; and it is just a question of degree whether the one or the other gets the mastery in this "tug of war." The more you heat the water, the faster the little groups of molecules break up and disappear in the air. They must in some way go moving between the particles of air, and collisions keep taking place with inconceivable rapidity. And now another question arises; namely, how much water-vapour can the air take? That depends chiefly on its temperature. Air when heated will take up a great deal of steam; and the more you heat air, the more it can take up. When air at a given temperature can take up no more, it is said to be saturated for that temperature; but if the temperature be raised, it will immediately begin to take up more. For each degree of temperature there is a certain amount of water-vapour which can be absorbed, and no more. But suppose we take some air which is already saturated and lower its temperature by giving it a sudden chill, what will happen? It will immediately give up part of its steam, or water-vapour; namely, the exact amount which it is unable to contain at the lower temperature.[8] [8] Pressure also has an important influence, but was omitted above for the sake of simplicity. There are various ways in which you can test this matter for yourself. For instance, take a hand-glass, and breathe on it. You know what will happen: a film of moisture forms upon it; and you know the reason why. It is simply that the cold glass gives a chill to one's breath (which being warm is highly charged with water-vapour from the lungs), and so some of the vapour is at once condensed. Now, this serves very well to explain how mountains catch water-vapour, and condense it. They are, as it were, a cold looking-glass; and the hot breath of the plains, as it strikes their sides, receiving a sudden chill, throws down part of the vapour it contains. On the higher parts of mountain-ranges the cold is so great that the water assumes the form of snow.