On the other hand the rock may succumb to changes which are purely mechanical and are due either to the stresses set up by differences between surface and interior temperatures, or to the prying action of the frost in the crevices. Such purely mechanical degeneration of the rocks is in contrast with decomposition and is described as _disintegration_. The two processes of decomposition and disintegration may, however, go on together; and the changes of volume that are caused by decomposition may result directly in considerable disintegration, as we are to see. =The rôle of the percolating water.=—In order to effect chemical change or reaction, it is essential that the substances which are to react must be brought into such intimate contact with each other as it is seldom possible to attain except by solution. The chemical reactions which go on between the gaseous atmosphere and the solid lithosphere are accomplished through solution of the gases in water. This water, derived from rain or snow, percolates into the ground or descends along the crevices in the rocks, carrying with it a certain measure of dissolved air. This air differs from that of the surrounding atmospheric envelope by containing relatively large amounts of oxygen and of the other active element carbon dioxide. It follows from the important rôle thus performed by the percolating water that the process of decomposition will be relatively important in humid regions where the atmospheric precipitation is sufficient for the purpose. [Illustration: FIG. 155.—Successive diagrams to show the effect of decomposition and resulting disintegration upon joint blocks so as to produce spheroidal bowlders by weathering.] Within hot and dry regions there is a larger measure of rock disintegration, and distinct chemical changes unlike those of humid regions take place in the higher temperatures and with the more concentrated saline solutions. The discussion of such changes will be deferred until desert conditions are treated in another chapter. =Mechanical results of decomposition—spheroidal weathering.=—From an earlier chapter it has been learned that the rocks of the earth’s outermost shell are generally intersected by a system of vertical fissures which at each locality tend to divide the rock into parallel and upright rectangular prisms. It is these joints which offer relatively easy paths for the descent of the water into the rocks. In rocks of sedimentary origin there are found, in addition to the vertical joints, planes of bedding originally horizontal, and in the intrusive and volcanic rocks a somewhat similar parting, likewise parallel to the surface of the ground. The combined effect of the joints and the additional parting planes is thus to separate the rock mass into more or less perfect squared blocks (Fig. 155, upper figure) which stand in vertical columns.