melting point, temperature at which a substance changes its state from solid to liquid. Under standard atmospheric pressure different pure crystalline solids will each melt at a different specific temperature; thus melting point is a characteristic of a substance and can be used to identify it. When heat is applied continuously and in sufficient quantity to such solids, the temperature rises steadily until it reaches the point at which liquefaction occurs. Here the rise ceases and no further change in temperature is observed until all of the substance has been converted to liquid. The heat being applied to the substance at that temperature is consumed in bringing about the change of state, and none is available to raise the temperature of that part of the substance already liquefied until all of it has changed to the liquid. If heat is still applied when liquefaction is complete, the temperature will begin to rise again. The quantity of heat necessary to change one gram of any substance from solid to liquid at its melting point is known as its latent heat of fusion and differs for different substances. Ice, for example, requires approximately 80 calories of heat to change each gram to water at its melting point. Because its heat of fusion is relatively high, ice is used in refrigeration. In freezing (the reverse process, i.e., the change from liquid to solid), heat is given off by the substance undergoing the change, and the amount given off is the same as that absorbed in melting.

interrogation point: see punctuation.

exclamation point: see punctuation.

dew point: see dew.

boiling point, temperature at which a substance changes its state from liquid to gas. A stricter definition of boiling point is the temperature at which the liquid and vapor (gas) phases of a substance can exist in equilibrium. When heat is applied to a liquid, the temperature of the liquid rises until the vapor pressure of the liquid equals the pressure of the surrounding gases. At this point there is no further rise in temperature, and the additional heat energy supplied is absorbed as latent heat of vaporization to transform the liquid into gas. This transformation occurs not only at the surface of the liquid (as in the case of evaporation) but also throughout the volume of the liquid, where bubbles of gas are formed. The boiling point of a liquid is lowered if the pressure of the surrounding gases is decreased. For example, water will boil at a lower temperature at the top of a mountain, where the atmospheric pressure on the water is less, than it will at sea level, where the pressure is greater. In the laboratory, liquids can be made to boil at temperatures far below their normal boiling points by heating them in vacuum flasks under greatly reduced pressure. On the other hand, if the pressure is increased, the boiling point is raised. For this reason, it is customary when the boiling point of a substance is given to include the pressure at which it is observed, if that pressure is other than standard, i.e., 760 mm of mercury or 1 atmosphere (see STP). The boiling point of a solution is always higher than that of the pure solvent; this boiling-point elevation is one of the colligative properties common to all solutions.

West Point, U.S. military post, since 1802 seat of the United States Military Academy. On the high west bank of the Hudson River N of New York City, West Point was the site of Revolutionary forts guarding the Hudson. Constitution Island, in the river, is also in the reservation. The plan of Benedict Arnold to surrender (1780) West Point to the British was discovered with the capture of Major John André.

Stevens Point, city (1990 pop. 23,006), seat of Portage co., central Wis., on the Wisconsin and Plover rivers; inc. 1858. The major industries are insurance and the manufacture of wood products, cheese, furniture, and fishing equipment. The Univ. of Wisconsin at Stevens Point is in the city.

Quonset Point, peninsula extending into Narragansett Bay, S R.I., in the town of North Kingstown.

Poverty Point National Monument: see National Parks and Monuments (table).

Point Roberts, uninc. town (1990 pop. 750), Whatcom co., NW Wash., on the Strait of Georgia near the tip of the Point Roberts peninsula, extending south from British Columbia, Canada, and separated from the Washington mainland by Boundary Bay. There is dairying, cattle raising, and tourism, as well as the manufacture of machinery parts and electronic components. The point was visited and named in 1792 by Captain George Vancouver. The town can be reached overland from the United States only by going through British Columbia, or by private boat. There is ferry service from nearby Tsawwassan to Swartz Bay, on Vancouver Island.

Point Reyes National Seashore, 71,068 acres (28,772 hectares), W Calif.; est. 1962. Included in the area are steep bluffs overlooking the Pacific Ocean, lagoons, and esteros enclosed by sand dunes, rolling hills, and forests. On offshore rocks are bird rookeries and sea-lion herds. The San Andreas fault passes through the park; there is a 15 to 20 ft (4.6-6.1 m) horizontal displacement of rock (a result of the 1906 earthquake). Sir Francis Drake probably stopped there (1579) to repair his ship, the Golden Hind, before crossing the Pacific Ocean. See National Parks and Monuments (table).

Point Pleasant, residential and resort borough (1990 pop. 18,177), Ocean co., E N.J., near the Manasquan Inlet; settled 1850, inc. 1920. Nearby Point Pleasant Beach (1990 pop. 5,112) is a summer seaside resort.

Point Pelee, peninsula, c.10 mi (16 km) long, extending into W Lake Erie, S Ont., Canada, near Leamington. It is the southernmost part of the Canadian mainland. Point Pelee National Park (6 sq mi/15.5 sq km; est. 1918) is there. The park is a wildlife sanctuary (especially for migratory birds) and is noted for its cattail marsh, woodland, and sandy beaches.

Point Judith, promontory extending into Block Island Sound, S R.I., in the town of Narragansett. A U.S. Coast Guard station and a lighthouse are there.

Point Grey, suburb of Vancouver, SW British Columbia, Canada. It is the site of the Univ. of British Columbia.

Point Four program, U.S. foreign aid project aimed at providing technological skills, knowledge, and equipment to poor nations throughout the world. The program also encouraged the flow of private investment capital to these nations. The project received its name from the fourth point of a program set forth in President Truman's 1949 inaugural address. In the cold war the U.S. government used Point Four to win support from uncommitted nations. From 1950 until 1953, Point Four aid was administered by the Technical Cooperation Administration, a separate unit within the Dept. of State. During the administration of President Eisenhower it was integrated into the overall foreign aid program.

Point Barrow, northernmost point of Alaska, on the Arctic Ocean, at lat. 71°23'N and long. 156°30'W. Visited in 1826 by Frederick W. Beechey, a British explorer, and named by him for the British geographer Sir John Barrow, it has since been the object of many expeditions and has figured prominently in arctic aviation. Navigation is open for only two or three months a year. To the southwest is the village of Barrow. Farther south is a monument to Will Rogers and Wiley Post, who lost their lives there in an airplane crash in 1935.

Pelee, Point: see Point Pelee, Ont., Canada.

Montauk Point, eastern extremity of the south peninsula of Long Island, SE N.Y. Approximately 115 mi (190 km) E of Manhattan, it is the easternmost point of the state. It has been the site of a lighthouse since 1795. The area is included in Montauk Point State Park.

High Point, city (1990 pop. 69,496), Davidson, Guilford, and Randolph counties, N N.C., in a heavily forested Piedmont region; settled before 1750, inc. 1859. It is a trade, industrial, and commercial center for an agricultural area. Noted for the production of furniture and hosiery, the city has become a major center for discounted furniture sales. Semiannual furniture expositions are held in High Point. Of interest is the restored home of a blacksmith (1786). High Point Univ. is in the city, as is the International Home Furnishings Center, the Furniture Discovery Center, and a museum on furniture manufacture.

Fort Point National Historic Site: see National Parks and Monuments (table).

Crown Point, town (1990 pop. 1,963), Essex co., NE N.Y., on Lake Champlain. Crown Point is a summer resort on a historic site. The French began building Fort St. Frédéric in 1731. In the French and Indian Wars the fort successfully resisted (1755-56) early English attacks but was demolished (1759) before the advance of Jeffery Amherst. The British began a new fort, Fort Amherst (renamed Crown Point) in 1759. Early in the American Revolution, Crown Point was captured (May 12, 1775) by Seth Warner and a detachment of Green Mountain Boys, but it was abandoned (June 22, 1777) to Gen. John Burgoyne in the Saratoga campaign. Crown Point Reservation, with bathing and fishing facilities, a museum, and ruins of colonial forts, is nearby.

Arguello, Point, promontory, SW Calif., extending W into the Pacific Ocean. A U.S. navy missile-launching complex is nearby.

Vibrational energy retained by molecules even at a temperature of absolute zero. Since temperature is a measure of the intensity of molecular motion, molecules would be expected to come to rest at absolute zero. However, if molecular motion were to cease altogether, the atoms would each have a precisely known location and velocity (zero), and the uncertainty principle states that this cannot occur, since precise values of both position and velocity of an object cannot be known simultaneously. Thus, even molecules at absolute zero must have some zero-point energy.

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or silverpoint

Method of drawing with a small sharpened metal rod—of lead, copper, gold, or most commonly silver—on specially prepared paper or parchment. Silverpoint produces a fine gray line that oxidizes to a light brown; the technique is best suited for small-scale work. It first appeared in medieval Italy and achieved great popularity in the 15th century. Albrecht Dürer and Leonardo da Vinci were its greatest exponents. It went out of fashion in the 17th century with the rise of the graphite pencil but was revived in the 18th century by the miniaturists and in the 20th century by Joseph Stella.

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Temperature at which the solid and liquid states of a pure substance can exist in equilibrium. As heat is applied to a solid, its temperature increases until it reaches the melting point. At this temperature, additional heat converts the solid into a liquid without a change in temperature. The melting point of solid water (ice) is 32°F (0°C). Though the melting point of a solid is generally considered to be the same as the freezing point of the corresponding liquid, they may differ because a liquid may freeze into different crystal systems and impurities can lower the freezing point.

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Temperature at which a liquid becomes a solid. When the pressure surrounding the liquid is increased, the freezing point is raised. The addition of some solids can lower the freezing point of a liquid, a principle used when salt is applied to melt ice on frozen surfaces. For pure substances, the freezing point is the same as the melting point. In mixtures and certain organic compounds, the early solid formation changes the composition of the remaining liquid, usually steadily lowering its freezing point, a principle that is applied in mixture separation. The freezing point of pure water at standard atmospheric pressure is 32°F (0°C). To change a liquid at its freezing point to a solid at the same temperature, the heat of fusion (see latent heat) must be removed.

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In science, the set of conditions under which a liquid and its vapour become identical. The conditions are the critical temperature, the critical pressure, and the critical density. If a closed vessel is filled with a pure substance, partly liquid and partly vapour, and the average density equals the critical density, the critical conditions can be achieved. As the temperature is raised, the vapour pressure increases, and the gas phase becomes denser while the liquid expands and becomes less dense. At the critical point, the densities of liquid and vapour become equal, eliminating the boundary between the two.

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Temperature at which a liquid is converted to vapour when heated. At the boiling point, addition of heat results in the transformation of the liquid into its vapour without an increase in temperature. A liquid's boiling point varies according to the liquid's characteristics and the applied pressure. Water at standard atmospheric pressure, or sea level, boils at 212 °F (100 °C), while ethanol boils at about 172 °F (78 °C). At higher altitudes, boiling points are lower and foods can take longer to cook; pressure cookers can be used to increase the pressure so that the boiling point is raised.

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