Nature never leaves them alone, never gives them a brief armistice in the long war that she wages against them. She is a relentless enemy, ever on the move, and ever varying her methods of attack. Now she assails them openly with her storm-clouds, and pelts them furiously with driving rain; now we hear the thunder of her artillery, as she pierces their crests with strange electric darts of fire; now she secretly undermines their sides with her hidden sources of water, till whole villages are destroyed by some fearful fall of overhanging rocks (see chapter iii., pages 96-101). Her winds and gentle breezes are for ever at work on their surfaces, causing them to crumble into dust much in the same way as iron turns to rust. Again, she heats them by day and then chills them suddenly at night, under the cold starry sky, so that they crack under the strain of expanding and contracting. Now she splits them with her ice-wedges; now she furrows their sides with the dashing torrents and running streams; and yet again she wears them gently down with her glaciers, and carries away their débris--the token of her triumph--on those icy streams, as conquering armies carry the spoils in procession. This is, briefly, her mode of warfare; these are some of her tools, _wind_, _rain_, _frost_, _snow_, _heat_ and _cold_, _streams_, _rivers_, and _glaciers_. Lightning does occasionally break off portions of a cliff or a mountain-peak; but compared to the others, this agent is not very important. Let us first inquire into the effects produced by the atmosphere. The air around us is composed mainly of two well-known gases; namely, oxygen and nitrogen. There is also a small proportion (about one in ten thousand) of carbonic acid gas; a variable quantity of water-vapour, and in the neighbourhood of towns, traces of other noxious gases, such as sulphurous acid and chlorine. Now, the nitrogen plays a very unimportant part, as it merely serves to dilute the powerful gas, oxygen, which has such important life-sustaining properties. We live by breathing oxygen; so do all animals; and the more pure air we can contrive to get into our lungs, the better. But undiluted oxygen would be too strong for us, and so its strength is diminished by being mixed with four parts of nitrogen; that is to say, the air only contains about one fifth by volume, or bulk, of oxygen and four fifths of nitrogen.