masses of solid ice, killing all animal and plant life. But the ice is lighter than water and floats on top, and animals in the water beneath are as free to live and swim as they were in the warm sunny days of summer. The most severe winter cannot freeze a deep lake solid, and in the coldest weather a hole made in the ice will show water beneath the surface. Our ice boats cut and break the ice of the river, and through the water beneath our boats daily ply their way to and fro, independent of winter and its blighting blasts.
While most of us are familiar with the bursting of water pipes on a cold night, few of us realize the influence which freezing water exerts on the character of the land around us.
Water sinks into the ground and, on the approach of winter, freezes, expanding about one tenth of its volume; the expanding ice pushes the earth aside, the force in some cases being sufficient to dislodge even huge rocks. In the early days in New England it was said by the farmers that "rocks grew," because fields cleared of stones in the fall became rock covered with the approach of spring; the rocks and stones hidden underground and unseen in the fall were forced to the surface by the winter's expansion. We have all seen fence posts and bricks pushed out of place because of the heaving of the soil beneath them. Often householders must relay their pavements and walks because of the damage done by freezing water.
The most conspicuous effect of the expansive power of freezing water is seen in rocky or mountainous regions (Fig. 21). Water easily finds entrance into the cracks and crevices of the rocks, where it lodges until frozen; then it expands and acts like a wedge, widening cracks, chiseling off edges, and even breaking rocks asunder. In regions where frequent frosts occur, the destructive action of water works constant changes in the appearance of the land; small cracks and crevices are enlarged, massive rocks are pried up out of position, huge slabs are split off, and particles large and small are forced from the parent rock. The greater part of the debris and rubbish brought down from the mountain slopes by the spring rains owes its origin to the fact that water expands when it freezes.
34. Heat Necessary to Dissolve a Substance. It requires heat to dissolve any substance, just as it requires heat to change ice to water. If a handful of common salt is placed in a small cup of water and stirred with a thermometer, the temperature of the mixture falls several degrees. This is just what one would expect, because the heat needed to liquefy the salt must come from somewhere, and naturally it comes from the water, thereby lowering the temperature of the water. We know very well that potatoes cease boiling if a pinch of salt is put in the water; this is because the temperature of the water has been lowered by the amount of heat necessary to dissolve the salt.
Let some snow or chopped ice be placed in a vessel and mixed with one third its weight of coarse salt; if then a small tube of cold water is placed in this mixture, the water in the test tube will soon freeze solid. As soon as the snow and salt are mixed they melt. The heat necessary for this comes in part from the air and in part from the water in the test tube, and the water in the tube becomes in consequence cold enough to freeze. But the salt mixture does not freeze because its freezing point is far below that of pure water. The use of salt and ice in ice-cream freezers is a practical application of this principle. The heat necessary for melting the mixture of salt and ice is taken from the cream which thus becomes cold enough to freeze.
CHAPTER IV
BURNING OR OXIDATION
35. Why Things Burn. The heat of our bodies comes from the food we eat; the heat for cooking and for warming our houses comes from coal. The production of heat through the burning of coal, or oil, or gas, or wood, is called combustion. Combustion cannot occur without the presence of a substance called oxygen, which exists rather abundantly in the air; that is, one fifth of our atmosphere consists of this substance which we call oxygen. We throw open our windows to allow fresh air to enter, and we take walks in order to breathe the pure air into our lungs. What we need for the energy and warmth of our bodies is the oxygen in the air. Whether we burn gas or wood or coal, the heat which is produced comes from the power which these various substances possess to combine with oxygen. We open the draft of a stove that it may "draw well": that it may secure oxygen for burning. We throw a blanket over burning material to smother the fire: to keep oxygen away from it. Burning, or oxidation, is combining with oxygen, and the more oxygen you add to a fire, the hotter the fire will burn, and the faster. The effect of oxygen on combustion may be clearly seen by thrusting a smoldering splinter into a jar containing oxygen; the smoldering splinter will instantly flare and blaze, while if it is removed from the jar, it loses its flame and again burns quietly. Oxygen for this experiment can be produced in the following way.
36. How to Prepare Oxygen. Mix a small quantity of potassium chlorate with an equal amount of manganese dioxide and place the mixture in a strong test tube. Close the mouth of the tube with a one-hole rubber stopper in which is fitted a long, narrow tube, and clamp the test tube to an iron support, as shown in Figure 22. Fill the trough with water until the shelf is just covered and allow the end of the delivery tube to rest just beneath the hole in the shelf. Fill a medium-sized bottle with water, cover it with a glass plate, invert the bottle in the trough, and then remove the glass plate. Heat the test tube very gently, and when gas bubbles out of the tube, slip the bottle over the opening in the shelf, so that the tube runs into the bottle. The gas will force out the water and will finally fill the bottle. When all the water has been forced out, slip the glass plate under the mouth of the bottle and remove the bottle from the trough. The gas in the bottle is oxygen.
Everywhere in a large city or in a small village, smoke is seen, indicating the presence of fire; hence there must exist a large supply of oxygen to keep all the fires alive. The supply of oxygen needed for the fires of the world comes largely from the atmosphere.
37. Matches. The burning material is ordinarily set on fire by matches, thin strips of wood tipped with sulphur or phosphorus, or both. Phosphorus can unite with oxygen at a fairly low temperature, and if phosphorus is rubbed against a rough surface, the friction produced will raise the temperature of the phosphorus to a point where it can combine with oxygen. The burning phosphorus kindles the wood of the match, and from the burning match the fire is kindled. If you want to convince yourself that friction produces heat, rub a cent vigorously against your coat and note that the cent becomes warm. Matches have been in use less than a hundred years. Primitive man kindled his camp fire by rubbing pieces of dry wood together until they took fire, and this method is said to be used among some isolated distant tribes at the present time. A later and easier way was to strike flint and steel together and to catch the spark thus produced on tinder or dry fungus. Within the memory of some persons now living, the tinder box was a valuable asset to the home, particularly in the pioneer regions of the West.
38. Safety Matches. Ordinary phosphorus, while excellent as a fire-producing material, is dangerously poisonous, and those to whom the dipping of wooden strips into phosphorus is a daily occupation suffer with a terrible disease which usually attacks the teeth and bones of the jaw. The teeth rot and fall out, abscesses form, and bones and flesh begin to decay; the only way to prevent the spread of the disease is to remove the affected
