212 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME 14. What is standard teinpcraturf? 15. What are the freezing- and boiling-points of water in the three temperature scales? 16. What is the normal temperature of the body on the centigrade and absolute scales? 17. What is the average room temperature (68° F".) on the centigrade scale? 18. What is absolute zero? 19. On what basis was the absolute scale worked out? 20. Give the principle of the aneroid barometer. 21. How would you construct a mercurial barometer?
UNIT IV SECTION 2 A KNOWLEDGE OF THE PROPERTIES OF LIQUIDS HAS LED TO MANY USEFUL APPLICATIONS Introduction. If the temperature could be raised sufficiently, all matter could be changed to the gaseous state. This is the actual condition of the matter in the sun. It is also generally recognized that nearly all solids may be changed to the liquid state by raising the temperature, provided that they do not decompose or sublime first. The great abundance of matter in a liquid state (such as water) and the fact that liquids are more tangible than gases caused some of their properties to be studied thousands of years ago, and several of the applications studied in this Section have been known for a long time. Viscosity. Liquids possess the property of fluidity, which is a measure of the ease with which they flow. Viscosity is a universal property of liquids. It is the inverse of fluidity, for it is measured by the resistance to flow The viscosities of such resulting from the internal friction of a liquid. liquids as molasses and tar are high at ordinary temperatures, while the viscosities of water and alcohol are much lower. It is a common observation that tar is heated in order to make it is flow readily, and it a general rule that viscosities of liquids decrease with increases in temperature. Some liquids have such high viscosities that there is a question whether they are liquids or solids. A true crystalline solid ceases to be crystalline when it flows. True solids are distinguished from highly viscous liquids in that true solids have sharp melting-points and produce characteristic patterns in the X-ray spectrograph. Therefore, glass tubing which will bend out of shape in a few weeks at ordinary temperatures and a paraffin candle which droops in the summer weather are properly regarded as examples of highly viscous liquids. Lubricating oil is purchased by the S.A.E. number, which refers to the viscosity measured by standard methods prescribed by the Society 213