6. Temperature-Absolute Zero.-One of the properties of heat is called temperature; this property can be measured by a thermometer and is proportional to the intensity of the heat. All our knowledge of heat, as obtained by the sensation of feeling, deals only with the temperature, and the terms in common use by means of which bodies are compared and denominated hot, hotter, hottest, have reference, not to the heat actually in the different bodies, but to the temperature.

There is always a tendency for heat to flow through intervening mediums from a hotter to a colder body, and there is no tendency for heat to flow from a cold to a hot body, although the relative amounts of heat in the two bodies might be different from that indicated by the thermometer. Thus, as an illustration, a pound of water requires about eight times as much heat to raise it one degree in temperature as a pound of iron, and hence when equal weights of both of these materials. are at the same temperature the water contains eight times as much heat as the iron, although in common parlance the two bodies would be equally hot.

The tendency for the hotter body to cool off and give up its heat to surrounding objects is characteristic of all materials, and if no other heat were supplied all bodies would come sooner or later to one common temperature. This temperature, when finally reached by all bodies in the universe, will represent the ultimate limit of all cooling and almost the entire absence of heat. It will be near absolute zero for all thermometric scales, and no greater cold will be possible or even conceivable. The inter-planetary space is believed by many to be very nearly at this limit, at the present time. Scientific men have made very careful determinations to ascertain what such a temperature must be, compared with the ordinary thermometric scales.

A perfect gas which remains under constant pressure will contract in volume an amount directly proportional to the change of temperature when reckoned from the point of greatest cold, which point is known as the absolute zero. By experiment it is found that when air is at a temperature of 32 degrees its volume is reduced one part in 492 each time that the temperature is lowered one degree. From this fact it has been concluded that the absolute zero is 492 degrees on the Fahren

heit scale or 273 degrees on the Centigrade scale, below the freezing-point of water. Strictly speaking there is no perfect gas, yet the results obtained with different gases by different observers are so nearly in accord that there is no question but that the results as given above are for all practical purposes correct.

7. Thermometer Scales.-The thermometer is an instrument used to measure temperature. The effect of heat is to expand or to increase the volume of most bodies. For perfect gases the amount of this expansion is strictly proportional to the change of temperature; for liquids and solids the expansion, while not exactly proportional to the increase of temperature, is very nearly proportional to it, and these bodies can be used for an approximate and even a close measure of difference of temperature. In nearly all thermometers the temperature is measured by the expansion of some body, mercury, alcohol, or air being commonly used as the thermometric substance.

The first thermometer was probably made by Galileo before 1597. It consisted of a glass bulb containing air, terminated below in a long glass tube which dipped into a vessel containing a colored fluid. The variations of volume of the enclosed air caused the fluid to rise or fall in the tube, the temperature being read by an arbitrary scale. Alcohol thermometers were in use as early as 1647, being made by connecting a spherical bulb with a long glass stem, on which graduations were made by beads of blue enamel placed in positions corresponding to one thousandths of the volume.

Fahrenheit, a German merchant, in 1721 was the first to make a mercurial thermometer, and the instrument which he designed, with certain modifications, has been retained in use by the English-speaking people up to the present time. Fahrenheit took as fixed points the temperature of the human body, which he called 24 degrees, and a mixture of salt and sal-ammoniac, which he supposed the greatest cold possible, as zero. this scale the freezing-point is 8 degrees. These degrees were afterwards divided into quarters, and AL THERlater these subdivisions themselves, termed degrees. On this modified scale the freezing-point of water becomes 32

On

FIG. I.
ORDINARY

FORM OF
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degrees, blood-heat 96* degrees, and the point of boiling water at atmospheric pressure 212 degrees. Unscientific as this thermometer is, it has been retained by two of the principal nations of the world, the English and the American; it is awkward to use, it was borrowed from a foreign nation which had itself adopted a more scientific instrument, and except for the fact that it has been long in use it has not a single feature to recommend it.

In 1724 Delisle introduced a scale in which the boilingpoint of water was called zero and the temperature of a cellar in the Paris Observatory was called 100 degrees. This thermometer was used for many years in Russia, but is now obsolete. In 1730 Réaumur made alcohol thermometers in which the boiling-point of water was marked 80 degrees. This thermometer is still in use in Russia.

Celsius adopted a centesimal scale in 1742 on which the boiling-point was marked zero and the freezing-point of water 100 degrees. This instrument is not now in use, although the centigrade scale is often called after Celsius. The botanist Linnæus introduced the centigrade thermometer, in which the freezing point of water is marked zero and the boilingpoint of water 100 degrees. This themometer is now adopted for ordinary use by the nations of continential Europe and for scientific use by every nation in the world.

The relative values of the degrees on the different thermometers used by various nations are given in the following table:

In all thermometric scales as given above, fixed points are determined by reference to the freezing and boiling points of water, with barometer at 29.92 inches, and all thermometers are constructed by marking these two points and then subdividing into the required number of degrees. The boilingpoint of water changes with the atmospheric pressure and with the purity of the water. The greater the pressure the higher the boiling-temperature. A table in the Appendix of this book shows the relation between the barometer pressure and the temperature of boiling water at atmospheric pressure. Mercury, alcohol, liquids and solids generally do not expand uniformly for each degree of temperature, or, in other words, they are not perfect thermometric substances. The error, however, is slight and is of more scientific than practical importance. Any perfect gas, however, does expand uniformly and is a perfect thermometric substance, but gas varies in volume with slight change in barometric pressure, and, while of great value as material for a scientific thermometer, is too bulky and awkward for ordinary use. It is at the present time considered doubtful if there is any perfect gas in existence, or one which cannot be liquefied by intense cold or great pressure. Air, hydrogen, and nitrogen act like perfect gases at ordinary temperatures; the same is true in a slightly less degree of oxygen. Yet oxygen is a liquid whose boilingpoint is 119 degrees centigrade (182 degrees Fahrenheit) below zero. Nitrogen and air are liquids boiling at a temperature of 193 degrees centigrade (315 degrees Fahrenheit) below zero. Pictet and Cailletet have reduced the temperature to 200 degrees C. below zero, finding air at that temperature to be a liquid as limpid as water and, like water, having a decided blue tint when seen by transmitted light.

8. Special Forms of Thermometers.-The mercurial thermometers, as ordinarily constructed (Fig. 1), consist of a bulb of glass joined to a capillary glass tube filled so as to leave a vacuum in the upper part of the glass stem, above the mercury; they cannot be used for any temperature higher than that of the boiling-point of mercury, which is about 575° F. More recently these thermometers have been filled with nitrogen or carbonic dioxide in the upper part of the glass stem,

which by pressure prevents the mercury boiling. Thermometers constructed in this way can be used safely in temperatures as high as the melting-point of ordinary glass, say to 1000° F. Mercurial thermometers are made in various ways; the cheaper ones have graduations on an attached frame of wood or metal, Fig. 1, but the more accurate and better grades have the graduations cut directly on to the glass stem, Fig. 2. It has been found that the glass from which these thermometers are made changes volume slowly for many months after construction, so that it is necessary to fill the thermometer with mercury a long time before graduation. In the better grade of thermometers the graduations are obtained by comparing point by point with an accurate standard; in the cheaper ones by simply subdividing into equal parts between freezing and boiling points. At very low temperatures (-38° F.) mercury solidifies and its rate of expansion changes; alcohol or spirits of similar nature are not so affected, and hence are better suited for use in thermometers for measuring extremely low temperatures. Air thermometers, while rather difficult to use and of somewhat clumsy construcFIG. 2 tion, are accurate through any range of temperature. These are made either by confining the air in a constant volume and measuring the increase in pressure (Fig. 3), or else by maintaining the pressure constant and noting the increase in volume. If the volume be maintained constant, the pressure will increase directly proportional to the increase in absolute temperature. In the air thermometer (Fig. 3) the volume is kept constant and the increase in pressure is measured by the rise of mercury in the tube OC above the line AB. That is, in passing from the freezing to the boil

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ing point of water, the barometer being FIG. 3. at 29.92, the pressure will increase 180/492, as expressed on the Fahr. scale, or 100/273 on the Cen. scale.