COEFFICIENTS OF CUBICAL EXPANSION OF GASES AT ATMOSPHERIC PRESSURE.
Air 0.003667 Carbon monoxide 0.003667 Hydrogen 0.003662 Carbon dioxide . 0 . 00372d Nitrogen 0 . 003668 Sulphur dioidde . 0. 003845 Oxygen 0.003668 on=== The Convection of Heat.— When the air in contact with a hot stove becomes warmed, it expands and grows lighter than the other air. Owing to unbalanced forces the hot air rises to the ceiling and then spreads out to the walls. It there becomes cooled and therefore contracts and becomes dense. As a result it descends at the walls and finally returns to the stove only to start again on the journey. During this process, called convection, heat is carried by the air from the stove to the most distant parts of the room. Winds consist of convec tion currents in the atmosphere. Some parts of the earth's surface become more highly heated by the sun than others. The air over the hot areas expands and becomes specifically lighter than the surrounding air. The general result is that the hot air is forced to rise, giv ing place to the surrounding cooler air, which blows toward the hot area as a surface wind. The hot air risen aloft spreads away toward the cool regions as an upper wind. Corresponding to the ascent of air over the hot areas is a de scent of air over the cool areas. Much heat is brought from the tropical regions to tem perate regions by regular winds.
Convection phenomena • also occur in liquids. A large vessel of water supplied with heat at one side of the bottom becomes through the action of convection currents uniformly heated throughout. Much heat is conveyed from the equator toward the poles by means of the Gulf Stream and other ocean currents. It is prob able, however, that with ocean currents differ ences of temperature have little to do with the motion of the water, but that the motion is caused chiefly by the action of winds that blow with great steadiness in a westerly direction across the equatorial portions of the great oceans. Difference in salinity of the ocean at different latitudes may possibly be a partial cause of the phenomenon.
Thermometry— Before proceeding further in the discussion of heat phenomena, it will be necessary to describe some of the methods em ployed for measuring temperature or the de gree of hotness of a body. Most commonly the methods depend upon the property of expan sion. In ordinary thermometers the expanding body is either mercury or colored alcohol. The liquid, say men:airy, is held in a glass tube hav ing a fine bore and at one end a spherical or cylindrical bulb, the other end being simply closed. Above the mercury, which fills the bulb and part of the stem, is a space that is free from air and contains only a amount of mercury vapor. When the thermometer is warmed, the mercury rises in the tube because the cubical expansion of mercury is greater than the cubical expansion of glass. The glass
tube is provided with a scale, sometimes engraved directly on the tube, and sometimes engraved on some other material and mounted at the back of the tube. For a Fahrenheit scale, divi sion number 32 is placed opposite the mercury level when the thermometer is placed in pure crushed melting ice, and division number 212 is placed opposite the mercury level when the thermometer is placed in saturated steam over boiling water. As the temperature of the boil ing point depends upon the atmospheric pres sure, which is ever varying, the standard' boil ing point is taken to correspond to the average atmospheric pressure, which is measured by a barometric column of 760 millimetres. The space between these marks, the freezing and boiling points, is divided into• 180 equal divi sions, and then divisions equal to these are extended above the boiling point and below the freezing point. For the Centigrade scale, which is generally employed in scientific work, the freezing point on the thermometer is marked 0° and the boiling point 100°. For the Riau iriur scale, much used for household purposes in Germany, these points are marked 0° and PO° respectively, and finally for the De Lisle scale, which is used in 'Russia, the direction 'of the graduation 'is reversed, the boiling point being marked 0° and the freezing point -I-•I50°. With this last thermometer, the greater the intensity of the cold the higher the number representing the temperature. Mercury thermometers per mit of the measurement of rather high tem peratures, mercury not boiling until the tem perature of about 357° C. (674.3 F.) is reached. Still higher temperatures with mercury ther mometers may be reached by checking the vaporization of the mercury through the in troduction into the upper part of the tube of a compressed gas, such as nitrogen. With such a thermometer the only limitation is the soften ing of the glass at high heats, and even this trouble is largely lessened by the use of vitrified quartz for the material of the bulb. On the other hand, mercury freezes at about — 39° C. (-38.2° F.) and so becomes useless for in dicating temperatures lower •than this. ' For these lower temperatures alcohol may be' em ployed as the thermometric substance because it resists freezing until temperatures far below any met with in nature are encountered. In addition to this advantage alcohol expands much more rapidly than Mercury, thus permit ting a much larger bore for the same length of degree. However, for very high temperatures alcohol is not available, as it boils at the mod erate temperature of 78.3° C. (173° F.).