Water which soaks into the ground moves slowly downward and after a longer or shorter journey, meets with a non-porous layer of rock through which it cannot pass, and which effectually hinders its downward passage.  In such regions, there is an accumulation of water, and a well dug there would have an abundant supply of water.  The non-porous layer is rarely level, and hence the water whose vertical path is obstructed does not “back up” on the soil, but flows down hill parallel with the obstructing non-porous layer, and in some distant region makes an outlet for itself, forming a spring (Fig. 38).  The streams originating in the springs flow through the land and eventually join larger streams or rivers; from the surface of streams and rivers evaporation occurs, the water once more becomes vapor and passes into the atmosphere, where it is condensed and again falls to the earth.

Water which has filtered through many feet of earth is far purer and safer than that which fell directly into the rivers, or which ran off from the land and joined the surface streams without passing through the soil.

72.  The Composition of Water.  Water was long thought to be a simple substance, but toward the end of the eighteenth century it was found to consist of two quite different substances, oxygen (O) and hydrogen (H.)

[Illustration:  FIG. 39.—­The decomposition of water.]

If we send an electric current through water (acidulated to make it a good conductor), as shown in Figure 39, we see bubbles of gas rising from the end of the wire by which the current enters the water, and other bubbles of gas rising from the end of the wire by which the current leaves the water.  These gases have evidently come from the water and are the substances of which it is composed, because the water begins to disappear as the gases are formed.  If we place over each end of the wire an inverted jar filled with water, the gases are easily collected.  The first thing we notice is that there is always twice as much of one gas as of the other; that is, water is composed of two substances, one of which is always present in twice as large quantities as the other.

73.  The Composition of Water.  On testing the gases into which water is broken up by an electric current, we find them to be quite different.  One proves to be oxygen, a substance with which we are already familiar.  The other gas, hydrogen, is new to us and is interesting as being the lightest known substance, being even “lighter than a feather.”

An important fact about hydrogen is that in burning it gives as much heat as five times its weight of coal.  Its flame is blue and almost invisible by daylight, but intensely hot.  If fine platinum wire is placed in an ordinary gas flame, it does not melt, but if placed in a flame of burning hydrogen, it melts very quickly.

74.  How to prepare Hydrogen.  There are many different methods of preparing hydrogen, but the easiest laboratory method is to pour sulphuric acid, or hydrochloric acid, on zinc shavings and to collect in a bottle the gas which is given off.  This gas proves to be colorless, tasteless, and odorless. (See Laboratory Manual.)