Now, oxygen, being always ready to combine chemically with some other element, is a great agent of change and decay. It attacks all the metals except gold and platinum. Iron, we all know, oxidises, or rusts, only too quickly; but copper, lead, silver, and other metals are more or less attacked by it. So it is with all the rocks exposed at or near the surface of the earth. Oxygen will, if it can, pick out something to combine with and so bring about chemical changes which lead to decay. But a much more powerful agent is the carbonic acid gas in the atmosphere; although there is so little of it, there is enough to play a very important part in causing rocks to crumble away, and in some cases to dissolve them entirely. The supply of this gas is continually being renewed, for all living animals breathe out carbonic acid, and plants give it out by night. Under the influence of sunlight plants give out oxygen, so that gas is supplied to the air by day. Both oxygen and carbonic acid gas are dissolved by rain as it falls through the air; and so we cannot separate the effects of the dry air by itself from those of rain and mist, which are more important agents. The action of rain is partly mechanical, partly chemical, for it not only beats against them, but it dissolves out certain mineral substances that they contain. All rocks are mixtures of two or more kinds of minerals, the particles of each being often invisible to the naked eye. Thus granites are essentially mixtures of felspar, quartz, and mica; ordinary volcanic rocks ("trap-rocks") of felspar and augite; sandstones consist mainly of particles of silica; limestones of carbonate of lime; shales and slates of silicate of alumina, the principal substance in clay. These grains are usually joined together by a cement of some mineral differing more or less from the other particles. Lime is found in many of the rocks as the cement that binds their particles together; while oxide of iron and silica serve this purpose in many other instances. Now, if the lime or iron or silica is dissolved by water, the rock must tend to crumble away. Any old building shows more or less manifold signs of such decay, and this process is called "weathering." All this applies merely to the surfaces of rocks; and if there were no other forces at work, their rate of decay would be very slow.