redness, or from the hand, is called obscure, or dark heat. But dark radiant heat obeys the same laws of motion as light and luminous heat.

FIG. 40.

92. Diathermancy.-Bodies which transmit heat freely are called diathermic; those which arrest it, athermic. Rock-salt (common salt in blocks) is the most perfect diathermic body, allowing all the heat-rays-those from the sun and the hand-to pass through with equal freedom. What glass is to light, a plate of rock-salt is to heat, and it has hence been aptly termed "the glass of heat." This substance is therefore adapted to make prisms and lenses for the concentration and dispersion of dark heat. If a heated ball be placed between a plate of glass and one of rock-salt, Fig. 40, and bits of phosphorus be laid upon stands beyond, though the salt be many times thicker than the glass, the dark heat passes freely through it, igniting the phosphorus, while it is quite arrested by the glass.

Glass intercepting, and Rock-Salt transmitting Heat.

93. Absorption of Heat by Aqueous Vapor.-Aqueous vapor, when condensed to minute particles of water, is highly opaque to the dark radiations. Where the atmospheric gases arrest one ray of obscure heat, the small proportion of water suspended in the air stops sixty or seventy rays. Luminous solar heat penetrates the air, and, falling upon the earth, is changed into obscure heat, which cannot be radiated back into space. The watery particles are thus the "barb" of the atmosphere which prevents the escape of the heat, and thus maintains the temperature of the earth. It follows that, if aqueous vapor were withdrawn from the air, the terrestrial heat would so quickly radiate away that the earth would soon become uninhabitable; the invisible watery element of the air is, therefore, the blanket

which keeps the world warm. In all those localities where the atmosphere is dry, the nightly loss of radiant heat is great, so that even in the burning desert of Sahara there is nocturnal freezing.

3. Changes of Molecular Aggregation.

94. Liquefaction.-Heat applied to solids overcomes their cohesion and changes them to liquids. That degree of temperature which is required to liquefy a substance is called its melting-point. From hundreds of degrees below zero up to thousands above, the various substances of Nature melt at different temperatures, showing that each requires its particular amount of heat-force to throw it into the liquid state.

95. Latent Heat.-In effecting this change, a certain amount of heat disappears, or seems to be used up in the process; and, as it can no longer be detected as sensible heat, it is spoken of as latent heat. If we take an ounce of ice at 32°, and one of water at 174°, and put them together, when the ice is melted, we shall have two ounces of water at 32°. The ounce of hot water has, therefore, lost 142° of its heat in melting the ice, which amount is the "latent heat" of the resulting water. The amount of heat thus consumed in altering the form of bodies, without raising their temperatures, is different in different cases.

96. Specific Heat.-If we expose equal weights of different substances to the same source of heat, they do not all receive it with equal readiness or in equal amounts; some will receive more than others. Water requires thirty times as much heat as mercury to raise an equal weight of it through the same number of degrees. Hence bodies are said to have different capacities for heat, and, as each substance seems to require a particular quantity for itself, that quantity is called its specific heat. The art of measuring the specific heat of bodies is called calorimetry.

97. Heat liberated by Freezing.-If the change of a solid to a liquid consumes force, the reverse change must produce it; the force therefore reappears as heat, upon freezing. As the thawing of snow and ice in spring is delayed by the large amount of heat that is expended in the forming of water, so the freezing processes of autumn are delayed, and the warm season prolonged, by the large quantities of heat that escape into the air, from the changing of water into ice.

98. Freezing Mixtures.-Advantage is taken of the absorption of heat in liquefaction to produce freezing mixtures, the most common example of which is salt and ice. In this case the salt melts the ice to unite with its water, which in turn dissolves the salt, so that both solids are changed to liquids. These changes require large amounts of heat, which is absorbed from surrounding bodies; the cold produced sinking the thermometer 40° below zero.

99. Ebullition. When water is gradually heated, minute bubbles are formed at the bottom of the vessel, which rise a little way, are crushed in, and disappear. These consist of vapor or steam, which is formed in the hottest part of the vessel, but, as they rise through the colder water above, are cooled and condensed. As the heating continues, these rise higher and higher until they reach the surface and escape into the air, producing that agitation of the liquid. which is called boiling or ebullition.

100. Boiling-Point.-The temperature at which this takes place is called the boiling-point, and it varies with different liquids and in different circumstances. It is slightly influenced by the nature of the containing vessel. To glass and polished metallic surfaces liquids adhere with greater force than to rough surfaces; and, before vaporization can occur, this adhesion must be overcome. Substances dissolved in a liquid also raise its boiling-point on account of their adhesion. Under ordinary circumstances, water boils at 212°, but, saturated with common salt, its

boiling-point is 224°. It has lately been shown that the amount of air dissolved in the water affects its boilingpoint, as it presses the watery particles asunder, and thus aids them to take on the gaseous state. Water purged of its air by long ebullition has been heated to 275° without boiling. When it did boil, the water was instantly changed into vapor with a loud explosion, the cohesion of its particles being suddenly overcome, like the snapping of a spring, by the repulsive power of the accumulated heat. But the most important circumstance that influences the boiling-point is the pressure of the atmosphere. This resists the rising vapor, and, as it fluctuates, the boilingpoint varies. The pressure becomes lighter as we ascend into the atmosphere, and the temperature of the boilingpoint is correspondingly diminished, so that boiling water is less hot in high altitudes than in low ones.

101. The Spheroidal State.-Water adheres to most surfaces, but heat destroys this attraction, and, if drops of it fall upon a red-hot plate of metal, they gather into spheroids, roll about, and evaporate very slowly. Fig. 41 represents a mass of water in the spheroidal state. In this

FIG. 41.

Spheroid of Water.

FIG. 42.

Its Explosion.

case the heat of the metal produces a layer of vapor which supports the drop, so that it does not touch the surface, but is driven about by currents of heated air. The temperature of the spheroid never reaches the boiling-point of the liquid, as the vapor, being a non-conductor, does not transmit the heat from the metal, and, besides, it is kept

cool by evaporation from its surface. If the temperature of the plate be allowed to fall to a point at which the water wets its surface, it will be suddenly scattered in a kind of explosive ebullition, Fig. 42.

102. Vaporization. The change of solids or liquids by the force of heat to vapor is called vaporization. Substances which are readily converted into vapor are said to be volatile, while those which are vaporized with difficulty are termed fixed or non-volatile. The slow formation of vapor from the surfaces of bodies is called evaporation. It goes on at all temperatures, even from the surface of ice and snow, but is rapidly increased as the temperature rises.

103. Heat of Vaporization.-A much larger amount of heat is spent in converting liquids into vapors than in changing solids to liquids, while the vapors are no hotter than the liquids from which they are formed. The heat has been consumed in producing the repulsive motion and the consequent enormous expansion of the gaseous body. If the liquid is exposed to the air, it is impossible to raise its temperature above its natural boiling-point. All the heat added after boiling commences is carried away by the vapor. Water boiling violently is not a particle hotter than that which boils moderately.

104. The quantity of heat which disappears during evaporation is very large. With the same intensity it takes 5 times as long to evaporate a pound of water as it does to raise it from freezing to boiling; it hence receives 5 times as much heat. If, therefore, 180° were required to boil the pound of water, nearly 1,000° are necessary to change it to vapor, and, being spent in producing the change of state, it of course disappears as sensible heat. This quantity is, therefore, the "latent" heat of steam. If the process be reversed, and the vapor be made to reassume the liquid form, the heat reappears. The condensation of a pound of steam will raise 5 pounds of water from the freezing to the boiling point.