"Hard" water and "soft" water are very different. The rain that falls and fills our cisterns is not softer or more delightful to use than the well water in some favoured regions. In it, soap makes beautiful, creamy suds, and it is a real pleasure to put one's hands into it. But in hard water soap seems to curdle, and some softening agent like borax has to be added or the water will chap the hands. There is little satisfaction in using water of this kind for any purpose.

Hard water was as soft as any when it fell from the sky; but the rain water trickled into the ground and passed through rocks containing lime. Some of this mineral was absorbed, for lime is readily soluble in water. Clear though it may be, water that has lime in it has quite a different feeling from rain water. Blow the breath into a basin of hard water, and a milky appearance will be noted. The carbonic acid gas exhaled from the lungs unites with the invisible lime, causing it to become visible particles of carbonate of lime, which fall to the bottom of the basin.

Nearly all well water is hard. So is the water of lakes and rivers and the ocean, for limestone is one of the most widely distributed rocks in the surface of the earth. Rain water makes its way into the earth's crust, absorbs mineral substances, and collects in springs which feed brooks and rivers and lakes. Wells are holes in the ground which bore into water-soaked strata of sand.

We gain something from the lime dissolved in hard water, for it is an essential part of our food. We must drink a certain amount of water each day to keep the body in perfect health. The lime in this water goes chiefly to the building of our bones. Plant roots take up lime in the water that mounts as sap through the plant bodies. We get some of the lime we need in vegetable foods we eat.

All of the kingdom of vertebrate animals, from the lowest forms to the highest, all of the shell-bearing animals of sea and land, require lime. Many of the lower creatures especially these in the sea, such as corals and their near relatives, encase themselves in body walls of lime. They absorb the lime from the sea water, and deposit it as unconsciously as we build the bony framework of our bodies.

All the bone and shell-bearing creatures that die on the earth and in the sea restore to the land and to the water the lime taken by the creatures while they lived. Carbonic acid gas in the water greatly hastens the dissolving of dead shells. Carbonic acid gas, whether free in the air, or absorbed by percolating water, hastens the dissolving of skeletons of creatures that die upon land. Then the raw materials are built again into lime rocks underground.

The lime rocks are the most important group in the list of rocks that form the crust of the earth. They are made of the elements calcium, carbon, and oxygen, yet the different members of this calcite group differ widely in composition and appearance. So do oyster shells and beef bones, though both contain quantities of carbonate of lime.

Calcite is a soft mineral, light in weight, sometimes white, but oftener of some other colour. It may be found crystallized or not. Whenever a drop of acid touches it, a frothy effervescence occurs. The drop of acid boils up and gives off the pungent odour of carbonic acid gas.

The reason that calcite is hard to find in rocks is that percolating water, charged with acids, is constantly stealing it, and carrying it away into the ocean. The rocks that contain it crumble because the limy portions have been dissolved out.

Some limestones resist the destructive action of water. When they are impregnated with silica they become transformed into marble, which takes a high polish like granite. Acids must be strong to make any impression on marble.

The thick beds of pure limestone that underlie the surface soil in Kentucky and in parts of Virginia sometimes measure several hundred feet in thickness, a single stratum often being twenty feet thick. They are all horizontal, for they were formed on sea bottom, and have not been crumpled in later time. The dead bodies of sea creatures contributed their shells and skeletons to the lime deposit on the sea bottom. Who can estimate the time it took to form those thick, solid layers of lime rock? The animals were corals, crinoids, and molluscs. Little sand and clay show in the lime rock of this period, before the marshes of the Carboniferous Age took the place of the ancient inland sea of the Subcarboniferous Period, the sedimentary accumulations of which we are now talking about.

The living corals one sees in the shallow water of the Florida coast to-day are building land by building up their limy skeletons. The reefs are the dead skeletons of past generations of these tiny living things. They take in lime from the water, and use it as we use lime in building our bones. In each case it is an unconscious process of animal growth—not a "building process" like a mason's building of a wall. Many people think that the coral polyp builds in this way. They give it credit for patience in a great undertaking. All the polyp does is to feed on whatever the water supplies that its digestive organs can use. It is like a sea anemone in appearance and in habits of life. It is not at all like an insect. Yet it is common to hear people speak of the "coral insect"! Do not let any one ever hear you repeat such a mistake.

Southern Florida is made out of coral rock, but thinly covered with soil. It was made by the growth of reef after reef, and it is still growing.

The Cretaceous Period of the earth's eventful history is named for the lime rock which we know as chalk. Beds of this recent kind of limestone are found in England and in France, pure white, made of the skeletons of the smallest of lime-consuming creatures, Foraminifera. They swarmed in deep water, and so did minute sponge animalcules and plant forms called Diatoms that took silica from the water, and formed their hard parts of this glassy substance. The result is seen in the nodules of flint found in the soft, snow-white chalk. Did you ever use a piece of chalk that scratched the black-board? The flint did it. Have you ever seen the chalk cliffs of Dover? When you do see them, notice how they gleam white in the sun. See how the rains have sculptured those cliffs. The prominences left standing out are strengthened by the flint they contain. Chalk beds occur in Texas and under our great plains; but the principal rocks of the age in America were sandstones and clays.