=34. Properties of Matter.=--Many differences in the physical properties of solids are due to differences between the cohesive force of different kinds of molecules. In some substances, the attraction is such that they may be rolled out in very thin sheets. Gold is the best example of this, sheets being formed 1/300,000 of an inch thick. This property is called _malleability_. In other substances the cohesion permits it to be drawn out into fine threads or wire. Glass and quartz are examples of this. This property is called _ductility_. In some, the cohesion makes the substance excessively _hard_, so that it is difficult to work or scratch its surface. The diamond is the hardest substance known. Some substances are _tough_, others _brittle_. These are tested by the ability to withstand sudden shocks as the blow of a hammer. Important Topics 1. Molecular forces in solids; (_a_) adhesion, (_b_) cohesion. 2. Elasticity, Hooke's Law. 3. Contraction on cooling. 4. Malleability, ductility, hardness, brittleness, etc. Exercises 1. Give an illustration of Hooke's Law from your own experience. 2. What devices make use of it? 3. Do solids evaporate. Give reasons. 4. When iron is welded, is cohesion or adhesion acting? 5. When a tin basin is soldered, is cohesion or adhesion acting? 6. Sometimes a spring is made more elastic by _tempering_ and made soft by _annealing_. Look up the two terms. How is each accomplished? 7. Review the definitions: solid, liquid, and gas. Why do these definitions mean more to you now than formerly? 8. If a wire is stretched 0.3 cm. on applying 4 kg. of force, what force will stretch it 0.75 cm? Explain. 9. How long will it take under ordinary conditions for a gas molecule to cross a room? Give reasons for your answer. 10. What is meant by the elastic limit of a body? 11. Without reaching the elastic limit, if a beam is depressed 4 mm. under a load of 60 kg., what will be the depression under a load of 400 kg.? Of 600 kg.? 12. Name three substances that possess elasticity of volume. 13. Give three examples of each; elasticity of (1) compression, (2) stretching, (3) torsion, (4) flexure. Review Outline: Introduction and Molecules Physics; definition, topics considered, physical and chemical changes. Science; hypothesis, theory, law. Knowledge; common, scientific. Matter; three states, molecular theory. Mass, weight, volume. Metric system; units, tables, equivalents, advantages. Evidences of molecular motions; gases (3), liquids (5), solids (3). Evidences of molecular forces; liquids (3), solids (many) special properties such as: elasticity, tenacity, ductility, hardness, etc. Hooke's law; applications. CHAPTER III MECHANICS OF LIQUIDS (1) THE GRAVITY PRESSURE OF LIQUIDS