22. Malleability. A substance that may be beaten or rolled into thin sheets is said to be malleable. Brass can be rolled into sheets thinner than the paper of this book. Gold leaf is so thin that it is transparent.

23. Ductility. A substance that can be drawn into wire is said to be ductile. Some metals possess great ductility. Platinum has been drawn into wire only 0.00003 of an inch thick. In order to do this, a small platinum wire was covered with silver, forming a compound cylinder, the silver surrounding the platinum much as the wood surrounds the graphite in a lead pencil. This cylinder was drawn into a very small wire, which had still a platinum center surrounded by silver; then the silver was dissolved by an acid which does not affect platinum, and the platinum was left as a wire of microscopic fineness.

Demonstration.

Take a piece of glass tubing about 10 cm. long by the ends and hold the middle in the flame of a Bunsen burner near the top. When it becomes cherry red remove it from the flame and draw it out with a steady pull. Prove that it is a tube by blowing through it when one end is under the surface of water.

24. Hardness is a relative property; there is no such thing as an absolutely hard or soft body. A body that can scratch or wear another is the harder of the two. Glass is harder than wax but softer than the diamond. The diamond is the hardest of all natural substances, and diamond dust is used to cut other stones. Brittleness must not be mistaken for hardness. Steel, which is hard, is tough; while glass, which is also hard, is brittle.

Steel is rendered very hard and brittle by being heated to a red heat and then being plunged into water. In order to render it serviceable for cutting tools, or for springs, it is

reheated slowly until it has the desired degree of hardness, which is indicated by its color, when it is again plunged into water or oil. This process is called tempering.

Iron may be rendered soft, or annealed, by cooling it gradually and evenly from a high temperature. This renders iron wire pliable, and if the same process is applied to glass, the strains are taken out and it is much less liable to crack on being heated.

A swiftly moving body will cut one that is at rest even if the latter is harder, as in the case of a soft iron disk rotating at a high speed, which is sometimes used to cut off hard steel bars. The cutting power of an emery or carborundum wheel depends both upon the hardness of the material and the high speed at which it is run. A buffing wheel used for polishing metals is an example of this action.

25. Crystallization. Some matter in the form of a solution has the property of forming crystals. Crystals may also be formed when a melted metal solidifies on cooling. Zinc shows this very plainly on account of the size of its crystals. If a bar of cast zinc is broken, not only can the crystals be seen, but a line of weakness will be shown wherever they meet from the sides of the mold.

By a saturated solution is meant one that will take up no more of the substance. When salt is put into water until, after thorough stirring, some of the salt is still not dissolved, the solution is said to be saturated. When such a solution begins to evaporate, crystals are formed. Raising the temperature of a solution usually causes more of the substance to be dissolved and crystals form when it cools.

Demonstrations.

Make a saturated solution of salt and put it in a beaker. Set this in a quiet place, and after a few hours you will find the surface of the liquid covered with little cubical

crystals of salt. Let the solution stand for twenty-four hours and groups of crystals will be floating on the surface. Lift one of these out, invert it, and you will find a beautiful little pyramid formed of salt cubes.

Make a saturated solution of salt and fill a teacup nearly full. Set the cup in a saucer and put them in some quiet place. In a few days the salt crystals will creep over the edge of the cup and form a coating upon the outside

and in the saucer.

Make a solution of potassium bichromate. Pour a small quantity on a clear glass plate, and with a small stick work the liquid into the form of a flat, round mass. Set it in a quiet place over night, and in the morning observe the crystals with a reading glass. Very beautiful slides can be made in this way for projection on the screen with the lantern.

26. Mass and Weight. The mass of body is the amount of matter there is in it, as determined by "weighing "the body in a lever balance. The weight of a body, though depending upon its mass, is a different thing, and the two words should not be confused. The weight of a body is the measure of the mutual attraction between the body and the earth. This attraction varies slightly in degree at different parts of the earth, while the amount of matter, or mass, in a pound of lead, for instance, is the same everywhere.

27. Measurements. - The modern study of physics, with the accurate knowledge obtained therefrom, depends largely

upon precise measurements, made in the units of space, mass, and time. We shall consider two systems of measurements, the English because it is in general use, and the French or metric because of its simplicity and of its growing use in all scientific work.

Scientists

28. The C. G. S. System of Measurement. working in various countries felt the inconvenience of having various standards of length, mass, and time, and finally adopted, for scientific work, the French system, more precisely known as the centimeter-gram-second or C. G. S. system. The convenience of this system is so great that it is taking the place of the foot-pound-second or F. P. S. system, used in Great Britain and the United States.

29. Space of One Dimension: Length. The French measures of length were devised in the latter part of the eighteenth century, by physicists who aimed to accomplish two things: first, to obtain a unit based upon a natural measure; and second, to take advantage of the convenience of the decimal scale. To secure the first result, they took for the meter, or unit of length, the ten-millionth part of the distance from the equator to the pole, measured on the meridian passing through Paris. Subsequent and more accurate measurements have shown that the distance as originally measured was not absolutely correct, and that the length of the standard meter is contained in the quadrant of the earth 10,000,880 times. While this prevents the meter from being the decimal part of a natural unit, it does not affect the value of the meter as a practical unit.

The original standard meter is a rod of platinum kept in the archives at Paris, and the distance between its ends at the temperature of melting ice is 1 meter. The tempera