Joaquin Garcia, Mexican author: b. Mexico City, 1825•, d. 1894. He contributed numerous biographical articles to the
universal de his toria y geografia.' His historical researches are most important and are a great storehouse of information. They include (Colecd6n de documentos para la historia de Mexico' (1858 66) ; (Historia eclesiaqica indiana, obra escrita a fines del siglo XVI por Francisco Geronimo Mendieta de la 6rden de San Francisco' (1870); and (Nueva coleccion de documentos para is historia de Mexico' (1886-92) ;
ICE, water in the solid state. When suf ficiently cooled, water loses its fluidity, and be comes filled with multitudes of needle-like crys tals belonging to the hexagonal system (see CRYSTAL), which increase and interlace until the whole mass becomes solidified. In nature, this change begins at the surface of the water and spreads gradually downward, so that the exact course of the freezing is not so easy to trace as it is in the laboratory, where the water can be uniformly cooled throughout its entire mass. When the freezing process is complete, the crys talline nature of the solid that results from it is not at all obvious. It is clearly visible, however, in snow-flakes, the hexagonal form is also evident. In a solid block of ice the crystalline structure can also be demonstrated by a method that was used by Tyndall, as a beautiful and instructive lecture experiment. The image of a slab of pure ice is thrown upon a screen by means of a projection lantern provided with a powerful electric light. At first nothing is seen, but very shortly the heat-rays passing through the ice cause it to melt internally, and the melt ing takes place according to the internal crys talline structure, which is gradually brought out upon the screen in great beauty. Six-sided stars, suggestive of the snow-crystals, appear, and these enlarge and become serrated at the edges as the electric beam gradually destroys the molecular architecture, the process continu ing until the ice has been again reduced to the liquid form.
Pure water normally freezes at a tempera ture which is denoted by 32° on the Fahrenheit scale, and by 0° on the Centigrade and Reaumur scales. It is possible, however, to cool pure water to a temperature considerably lower than this, if proper precautions are taken, without crystallization. As long ago as 1836, for ex ample, Gay-Lussac observed that water, when placed in a vessel and covered with a layer of oil, may be cooled to 10° F. without freezing. If the vessel be slightly shaken or jarred, how ever, solidification ensues at once.
Pressure has an effect upon the tempera ture at which water freezes. This effect was predicted, from theoretical considerations, by James Thomson, in 1849. Dewar has since measured its amount with much care, finding that the freezing temperature is lowered by 0.014° F. for each atmosphere of pressure. Small as this quantity is, it is of importance in some branches of physics. In 1858 Mousson, by the application of an enormous pressure, suc ceeded in reducing the freezing point to 4° below zero, Fahrenheit. The presence of dis solved substances in the water also depresses the freezing point. Sea-water, for example, freezes at about 27° F. (the•ice that is formed nearly free from salt), strong brine is used in the circulating pipes and cooling coils of refrigerating plants, since it can be cooled much below this temperature without freezing.
The effect of pressure in lowering the freez ing point is illustrated in the familiar process of making a snow-ball from damp snow — that is, from snow whose temperature isprecisely 32° F. Under the pressure of the hand, the freez ing point of the snow mass is lowered slightly, with the result that a partial melting of the crystals takes place. When the pressure is re moved, the freezing point rises to the normal, and the water that was formed by the pressure alone freezes again, and cements the mass to gether. (The superficial moisture, due to the warmth of the hand, is not here contemplated. The melting from this cause is a separate phe nomenon.) The slight but real plasticity of large masses of ice, such as are met with in glaciers, is probably related to this phenomenon of the variation of the freezing point by pres sure, but there is some difference of opinion among the authorities as to the precise way in which the slow downward flow of these ice masses is accomplished. The melting of ice by pressure, and its subsequent solidification upon the removal of the pressure, is known to physicists as ((regelation.)) Experiments that have been conducted in connection with precise thermometry, by Pernet and Marek, show that the temperature of melt ing ice is slightly different, according to the source of the ice, and the way in which it is treated; this variation being independent of the pressure, and probably due to unrecognized im purities. A variation in the melting point of as much as 0.164° F. has been observed; and in order to eliminate the effects of error from this cause, it is necessary, for the purposes of precise thermometry, to adopt a uniform mode of prqcedure in the treatment of the ice that is to be used for the establishment of the freezing point upon accurate thermometers. Consult Guillaume, Th ermornetr e de (Chap. 2).