[_Saladin grasps Nathan’s hand, and holds to the end of the conversation._ _Saladin._ I the judge?—I’m dust! I’m nothing! ’Tis Allah!—Nathan, now I understand; The thousand thousand years have not yet passed; The judge is not yet come; I must not place Myself upon his throne! I understand— Farewell, dear Nathan! Go.—Be still my friend. READINGS IN PHYSICAL SCIENCE. Abridged from Science Primer on Physical Science, by Prof. Geikie. III.—RIVERS AND GLACIERS. We have found that the water of the river is largely derived from springs, and that all spring-water contains more or less mineral materials dissolved out of the brooks. Every river, therefore, is carrying not merely water, but large quantities of mineral matters into the sea. It has been calculated, for instance, that the Rhine in one year carries into the North Sea lime enough to make three hundred and thirty-two thousand millions of oyster shells. This chemically-dissolved material is not visible to the eye, and in no way affects the color of the water. At all times of the year, as long as the water flows, this invisible transport of some of the materials of rocks must be going on. But let us now again watch the same river in flood. The water is no longer clear, but dull and dirty. You ascertained that this discoloration arises from mud and sand suspended in the water. You see that over and above the mineral matter in chemical solution, the river is hurrying seaward with vast quantities of other and visible materials. And thus it is clear that at least one great part of the work of rivers must be to transport the mouldered parts of the land which are carried into them by springs or by rain. But the rivers, too, help in the general destruction of the surface of the land. Of this you may readily be assured, by looking at the sides or bed of a stream when the water is low. Where the stream flows over hard rock, you find the rock all smoothed and ground away; and the stones lying in the water-course are all more or less rounded and smoothed. When these stones were originally broken by frosts or otherwise, from crags and cliffs, they were sharp-edged, as you can prove by looking at the heaps of blocks lying at the foot of any precipice, or steep bank of rock. But when they fell, or were washed into the river, they began to get rolled and rubbed, until their sharp edges were ground away, and they came to wear the smooth rounded forms which we see in the ordinary gravel. While the stones are ground down, they, at the same time, grind down the rocks which form the sides and bottom of the river-channel over which they are driven. You can even see in some of the eddies of the stream how the stones are kept moving round until they actually excavate deep round cavities, called pot-holes, in the solid rock.